release-23.1: server: avoid a race in the server controller

[blathers-crl]

Backport 1/1 commits from #108401 on behalf of @knz.

/cc @cockroachdb/release

---

Co-authored with @lidorcarmel

Prior to this patch, the Go race detector was complaining about racy concurrent accesses to serverStateUsingChannels.server, via (*serverStateUsingChannels) getServer() and (*channelOrchestrator) startControlledServer().

These racy accesses happened to be safe because the writes and reads to that field were correctly ordered around updates to an atomic bool (the started field). However, the race detector is not sufficiently sophisticated to detect this ordering and satisfy itself that the state transition can only happen once.

In order to silence the race detector (with no change in correctness), this patch replaces the atomic bool by a mutex, whose access semantics are properly understood by the race detector.

This change incidentally makes the code slightly easier to read and understand.

Supersedes #108371.
Fixes #107930.
Epic: CRDB-28893

---

Release justification: avoids test flakes

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is