kvserver: sep-raft-log: extract and test replica destruction engine logic #93241
Description
We should extract the interactions with the engine in destroyRaftMuLocked1 into a standalone method.
We should clarify which operation occurs on which engine in which order.
We need to document the invariants being upheld and how to recover them in the event of an ill-timed crash.
Epic: CRDB-220
Jira issue: CRDB-22234