Improve name of AccountDB.lock_changes()

[carver]

Maybe call end_previous_txn() or mark_new_transaction().

The point is that we need some explicit way to reset some things that need to be reset in between transactions. Currently, that means making sure that the account & storage journal cannot be rolled back past the beginning of the transaction, and that the cache warmth tracking is reset to empty.

Originally posted by @carver in https://github.com/ethereum/py-evm/pull/1986#discussion_r593492886