After #160565 merges, similarly support the lazily created log engine batches in other write paths that replicaAppBatch is responsible for (see all users of the batch that need a log engine access):

• replica destruction when applying a merge
• replica destruction when applying a config change that removes our replica
• log truncation

Nuances (maybe more during reviews):

• The log truncation path is for the tightly coupled truncations. It won't be used with separated engines. Need to consider whether we want to skip it, or support half-legally (only to unblock some testing).
• The replica destruction in config changes has a condition which would be good to get rid of.

Epic: CRDB-55220
Jira issue: CRDB-58685