kvserver: sep-raft-log: integrate splits, merges, conf changes

[tbg]

Splits and merges pose an additional challenge under a separate raft log since they change replica boundaries.

• splits have tricky handling for the right-hand side - there are many cases to consider¹: the RHS might not exist yet, it might exist as an uninited replica, and in either case the right-hand side might already have been removed from this store, in which case we might have to shorten the LHS during the split. None of this is new but integrating with sep-raft-log should be done carefully.
• splits, merges, conf change application should be synced (this avoids having to know how to apply them at init time, though maybe this could be lifted)
• merges need all replicas to be caught up, and durably so², i.e. another sync there.

We will surely discover a bit more complexity here than is known once we actually set out to doing this.

https://github.com/tbg/cockroach/blob/e292a73f2af0e55aa760cce168622732c0716ff6/pkg/kv/kvserver/replicas_storage.go#L238-L248

Epic: CRDB-220

Jira issue: CRDB-22242

Footnotes

1. https://github.com/cockroachdb/cockroach/blob/f19bb2a9899728f62379e2444bc1bb07bdf684e4/pkg/kv/kvserver/store_split.go#L234-L252 ↩

2. https://github.com/tbg/cockroach/blob/e292a73f2af0e55aa760cce168622732c0716ff6/pkg/kv/kvserver/replicas_storage.go#L568-L572 ↩

[blathers-crl]

cc @cockroachdb/replication