Improve in memory block tracking

[mattsse]

The tree state contains all valid blocks including all sidechains

reth/crates/engine/tree/src/tree/mod.rs

Lines 70 to 78 in 70a0cea

	pub struct TreeState {
	/// __All__ executed blocks by block hash.
	///
	/// This includes blocks of all forks.
	blocks_by_hash: HashMap<B256, ExecutedBlock>,
	/// Executed blocks grouped by their respective block number.
	///
	/// This maps unique block number to all known blocks for that height.
	blocks_by_number: BTreeMap<BlockNumber, Vec<ExecutedBlock>>,

we need to update this on FCU and also detect reorgs

reth/crates/engine/tree/src/tree/mod.rs

Lines 166 to 171 in 70a0cea

	// walk back the chain until we reach the canonical block
	while parent.hash != self.canonical_block_hash() {
	if parent.number == self.canonical_head().number {
	// we have a reorg
	todo!("handle reorg")
	}

with this data structure this no longer is trivial

we likely need an additional index that maps parent/child relationships, similar to

reth/crates/blockchain-tree/src/block_buffer.rs

Lines 21 to 24 in 70a0cea

	/// Map of any parent block hash (even the ones not currently in the buffer)
	/// to the buffered children.
	/// Allows connecting buffered blocks by parent.
	pub(crate) parent_to_child: HashMap<BlockHash, HashSet<BlockHash>>,

so can easily travers a chain