raft: make MsgApp.Entries in StateProbe empty

[pav-kv]

When the follower replication flow is in StateProbe, the leader will likely send MsgApps with log entries.

The downsides of this:

• the probes aren't subject to replication admission control, and can lead to overload
• the probes are constructed inside RawNode under Replica.mu, so MsgApp construction with entries may lead to IO under this mutex. We would like to minimize it: raft: fetch from log storage without holding Replica.mu #130955.

A simple fix is to make StateProbe messages outright empty. However, this would have a performance impact because it's optimized for the happy case when the first probe is successful (and so some entries are replicated immediately). In a general case, this reduces post-StateProbe replication latency by one roundtrip to the follower.

There are some ideas in etcd-io/raft#150 on how to optimize these roundtrips differently. The "hints" in MsgAppResp can be extended to carry more information, as well as embedded in the MsgVoteResp to eliminate the first post-election probing roundrip.

Jira issue: CRDB-42653