[txpool](https://github.com/ledgerwatch/erigon-lib/blob/main/txpool/pool.go)
expects an `OnNewBlock` update only after the DB transaction is
committed.
This fixes, for example, a nonce gap mis-detection in Hive test
"engine-cancun/Blob Transactions On Block 1, Cancun Genesis".
This PR separates ENGINE from Ethbackend. It makes it so:
1) EthBackend not a god class
2) We can abstract away engine API so that we can make it CL-like and
enable Consensus-Execution driven design
3) Objective is Json-RPC -> Engine Consensus Module -> Execution module.
