kvserver: improve Raft append behavior when follower is missing log entries

In a test cluster, where `kv.raft_log.synchronization.unsafe.disabled` was enabled (i.e. process crashes will lose Raft data), we saw a range become unavailable, stalling the workload. CPU was persistently spinning at 100% on the Raft leader (n2) and one follower (n6), with the following CPU profiles:

n2 (leader):


n6 (follower):


Notice the call to `raftLog.findConflictByTerm()` on the leader. This call only happens when the follower rejected a `MsgApp`, via `MsgAppResp` with `Reject: true`:

https://github.com/etcd-io/raft/blob/ee0fe9da492888b55fe183cf1a42931ad551ec6b/raft.go#L1339-L1459

This happens when the follower fails to append a set of log entries, e.g. because the follower is lacking a prefix of the log:

https://github.com/etcd-io/raft/blob/ee0fe9da492888b55fe183cf1a42931ad551ec6b/raft.go#L1738-L1770

The logic here also involves silently swallowing an error when attempting to read the term from storage (although we can't have hit this on the follower, because that would return a 0 term hint to the leader who would then not call `findConflictByTerm()`):

https://github.com/etcd-io/raft/blob/1df762940b8c309a27cfafb086d767c0c7e3f58f/log.go#L180-L187

It seems plausible that data loss induced by `kv.raft_log.synchronization.unsafe.disabled` somehow ended up with either an append loop or hitting a slow path (e.g. there is a fallback here to probing indexes one by one, although it does not seem like we hit it here), where the leader continually sends MsgApps to the follower, who in turn rejects them.

We should make sure the behavior here is sound, and improve observability when this happens.

Jira issue: CRDB-32732