logstore: introduce logstore.Engine

[tbg]

This introduces the concept of an Engine specific for use in a log
store. In other words, it's the beginnings of a logical separation of
the state machine and log engines, i.e. #16624.

For now, both continue to be backed by the same storage.Engine, and
LogEngine is not correctly used in all places. For example, snapshot
application hasn't yet been updated to account for the possibility
of two separate engines and writes a set of SSTS that is atomically
ingested into the single engine currently present but which logically
spans both engines (#93251).

Touches #93243.

Epic: CRDB-220
Release note: None

[cockroach-teamcity]

This change is 

[tbg]

We need to talk about whether this approach is the right one. I can't think of a good alternative off-hand and I think we should have some type safety, but if you continue on this approach you'll change the storage.Writer params in, e.g., SetHardState to a similar logstore.Writer interface, and now you have the problem that when we create the batch in handleRaftReady we need to wrap it in an extra interface (with, I think, an extra allocation), which is not great.

[tbg]

We chatted earlier and we'll hold off on this a bit; it's mostly cosmetic anyway. Our time is better spent elsewhere for now.