Add a new interface StateProcessor.ProcessParallel(...), it is a
copy of Process(...) right now.
This patch is a placeholder, we will implement BEP-130 based on it.

** modules of init, slot executer and dispatcher
BEP 130 parallel transaction execution will maintain a tx execution routine
pool, a configured number of slot(routine) to execution transactions.

Init is executed once on startup and will create the routine pool.
Slot executer is the place to execute transactions.
The dispacther is the module that will dispatch transaction to the right slot.

** workflow: Stage Apply, Conflict Detector, Slot, Gas...
  > two stages of applyTransaction
  For sequential execution, applyTransaction will do transaction execution  and
  result finalization.

  > Conflict detector
  We will check the parallel execution result for each transaction.
  If there is a confliction, the result can not be committed, redo will
  be scheduled to update its StateDB and re-run
  For parallel execution, the execution result may not be reliable(conflict), use
  try-rerun policy, the transaction could be executed more than once to get the correct result.
  Once the result is confirm, we will finalize it to StateDB.
  Balance, KV, Account Create&Suicide... will be checked
  And conflict window is important for conflict check.

  > Slot StateDB
  Each slot will have a StateDB to execute transaction in slot.
  The world state changes are stored in this StateDB and merged to the main StateDB
  when transaction result is confirmed. SlotState.slotdbChan is the current execute TX's slotDB.
  And only dirty state object are allowed to merge back, otherwise, there is a race condition
  of merge outdated stateobject back.

** others
gas pool, transaction gas, gas fee reward to system address
evm instance, receipt CumulativeGasUsed & Log Index,
contract creation, slot state,
parallel routine safety:
  1.only dispatcher can access main stateDB
  2.slotDB will be created and merged to stateDB in dispatch goroutine.

** workflow 2: CopyForSlot, redesign dispatch, slot StateDB reuse & several bugfix

  > simplifiy statedb copy with CopyForSlot
  only copy dirtied state objects
  delete prefetcher

** redesign dispatch, slot StateDB reuse...
  > dispatch enhance
  remove atomic idle, curExec... replace by pendingExec for slot.

  >slot StateDB reuse
  It will try to reuse the latest merged slotDB in the same slot.
  If reuse failed(conflict), it will try to update to the latest world state and redo.
  The reuse SlotDB will the same BaseTxIndex, since its world state was sync when it was created based on that txIndex
  Conflict check can skip current slot now.
  it is more aggressive to reuse SlotDB for idle dispatch
  not only pending Txs but also the idle dispatched Txs try to reuse SlotDB now.

** others
state change no needs to store value

add "--parallel" startup options
Parallel is not enabled by default.
To enable it, just add a simple flag to geth: --parallel
To config parallel execute parameter: --parallel.num 20 --parallel.queuesize 30
"--parallel.num" is the number of parallel slot to execute Tx, by default it is CPUNum-1
"--parallel.queuesize" is the maxpending queue size for each slot, by default it is 10

For example:
  ./build/bin/geth --parallel
  ./build/bin/geth --parallel --parallel.num 10
  ./build/bin/geth --parallel --parallel.num 20 --parallel.queuesize 30

** several BugFix
1.system address balance conflict
  We take system address as a special address, since each transaction will
  pay gas fee to it.
  Parallel execution  reset its balance in slotDB, if a transaction try to access
  its balance, it will receive 0. If the contract needs the real system address
  balance, we will schedule a redo with real system address balance

  One transaction that accessed system address:
  https://bscscan.com/tx/0xcd69755be1d2f55af259441ff5ee2f312830b8539899e82488a21e85bc121a2a

2.fork caused by address state changed and read in same block
3.test case error
4.statedb.Copy should initialize parallel elements
5.do merge for snapshot

** move .Process() close to .ProcessParallel()
** InitParallelOnce & preExec & postExec for code maintenance
** MergedTxInfo -> SlotChangeList & debug conflict ratio
** use ParallelState to keep all parallel statedb states.
** enable queue to same slot

** discard state change of reverted transaction
   And debug log refine

** add ut for statedb

…ch for parallel

this patch has 3 changes:
1.change default queuesize to 20, since 10 could be not enough and will cause more conflicts
2.enable slot DB trie prefetch, use the prefetch of main state DB.
3.disable transaction cache prefetch when parallel is enabled
  since in parallel mode CPU resource could be limitted, and paralle has its own piped transaction execution

4.change dispatch policy
  ** queue based on from address
  ** queue based on to address, try next slot if current is full
  Since from address is used to make dispatch policy,
  the pending transactions in a slot could have several different
  To address, so we will compare the To address of every pending transactions.

** use sync map for the stateObjects in parallel
** others
  fix a SlotDB reuse bug & enable it
  delete unnecessary parallel initialize for none slot DB.

…t, prefetch, fork

This is a complicated patch, to do some fixup

** fix MergeSlotDB
Since copy-on-write is used, transaction will do StateObject deepCopy before it writes the state;
All the dirty state changed will be recorded in this copied one first, the ownership will be
transfered to main StateDB on merge.
It has a potential race condition that the simple ownership transfer may discard other state changes
by other concurrent transactions.
When copy-on-write is used, we should do StateObject merge.

** fix Suicide
Suicide has an address state read operation.
And it also needs do copy-on-write, to avoid damage main StateDB's state object.

** fix conflict detect
If state read is not zero, should do conflict detect with addr state change first.
Do conflict detect even with current slot, if we use copy-on-write and slotDB reuse, same
slot could has race conditon of conflict.

** disable prefetch on slotDB
trie prefetch should be started on main DB on Merge

** Add/Sub zero balance, Set State
These are void operation, optimized to reduce conflict rate.
Simple test show, conflict rate dropped from ~25% -> 12%

**fix a fork on block 15,338,563
It a nonce conflict caused by opcode: opCreate & opCreate2
Generally, the nonce is advanced by 1 for the transaction sender;
But opCreate & opCreate2 will try to create a new contract, the
caller will advance its nonce too.

It makes the nonce conflict detect more complicated: as nonce is a
fundamental part of an account, as long as it has been changed, we mark
the address as StateChanged, any concurrent access to it will be considered
as conflicted.

** optimize conflict for AddBalance(0)
Add balance with 0 did nothing, but it will do an empty() check, and add
a touch event. Add on transaction finalize, the touch event will check if
the StateObject is empty, do empty delete if it is.

This patch is to take the empty check as a state check, if the addr state has
not been changed(create, suicide, empty delete), then empty check is reliable.

** optimize conflict for system address

** some code improvement & lint fixup & refactor for params

** remove reuse SlotDB
Reuse SlotDB was added to reduce copy of StateObject, in order to mitigate
the Go GC problem.
And COW(Copy-On-Write) is used to address the GC problem too. With COW enabled,
reuse can be removed as it has limitted benefits now and add more complexity.

** fix trie prefetch on dispatcher

Trie prefetch will be scheduled on object finalize.
With parallel, we should schedule trie prefetch on dispatcher, since
the TriePrefetcher is not safe for concurrent access and it is created & stopped
on dispatcher routine.

But object.finalize on slot cleared its dirtyStorage, which broken the later trie
prefetch on dispatcher when do MergeSlotDB.

No fundamental change, some improvements, include:
** Add a new type ParallelStateProcessor;
** move Parallel Config to BlockChain
** more precious ParallelNum set
** Add EnableParallelProcessor()
** remove panic()
** remove useless: redo flag,
** change waitChan from `chan int` to `chan struct {}` and communicate by close()
** dispatch policy: queue `from` ahead of `to`
** pre-allocate allLogs
** disable parallel processor is snapshot is not enabled
** others: rename...