kvserver: sep-raft-log: type safety for log and state engine

With the introduction of the separate raft log, we have a number of methods that operate on an engine which is assumed to always be (say) raft log. It would be nice to leverage the type system to make it clear that some methods only ever operate on one kind of engine.

Perhaps extensions of `storage.Read{Writ,}er`, etc, could be introduced and used in appropriate locations.