kvserver: sep-raft-log: don't touch unreplicated RHS keyspace in split eval

[tbg]

When evaluating a split command, we call WriteInitialReplicaState for the right-hand replica and send the result through raft. But we really only want the effects on the replicated keyspaces on the right hand side here. The unreplicated portions (raft state etc) should be written below-raft, on each store. This is a "nice cleanup" without separate raft log, but with the log separation it becomes more of a prerequisite, though we could work around it by "massaging" the batch below raft to remove the offending writes.

Epic: CRDB-220

Jira issue: CRDB-24789

[blathers-crl[bot]]

cc @cockroachdb/replication

[tbg]

I think we might be able to remove these writes, we already have the code downstream of raft that does the right thing:

https://github.com/cockroachdb/cockroach/blob/a9081374615abb6a06511bc35d29121fd14259af/pkg/kv/kvserver/store_split.go#L139-L153

This will affect production even when not separating the raft log though, so it should be done with a good amount of prudency.