cmd/geth/retesteth: use canon head instead of keeping alternate count

[holiman]

Retesteth was kind of illogical, when blocks were imported [1,2,3a,4,3b], it thought that 3b was the canon head (since Importblocks does not return an error if a side-block is imported).

This PR removes the separate blockNumber counter from the API, and uses the actual current head from blockchain when needed.

Requested by @winsvega , PTAL if this fixes your issues.

[winsvega]

thanks. now it return the right head of the blockchain

[winsvega]

aha. here is another issue.

block 4  has invalid transaction which is rejected. nonce = 3
rewind to block 2
import valid transaction with nonce = 2
mine block 3

and the mined block 3 has now 2 transactions. one with nonce 2 and the second one with nonce 3 that is now valid. but after rewind, the transaction pool should be cleaned.