kvserver: no raft snaps should be necessary without log truncations

See https://github.com/cockroachdb/cockroach/pull/87554 for discussion.

It should always be possible to add a voter without relying on the raft snapshot queue, assuming log truncations are disabled.

We're seeing that this isn't true, but only due to inefficiencies in the implementation.

[This comment](https://github.com/cockroachdb/cockroach/pull/87554#issuecomment-1408950303) motivates and makes two suggestions:

- address a TODO in raft to move out of StateSnap more freely
- also probe followers in StateSnapshot

Together they should achieve the stated goal, which the test in #87554 can help verify.

Jira issue: CRDB-23999