release-21.2: liveness: improve disk probes during node liveness updates

[erikgrinaker]

• Backport 1/1 commits from liveness: improve disk probes during node liveness updates #81133.
• Backport 2/2 commits from liveness: run sync disk write in a stopper task #81813.

/cc @cockroachdb/release

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When NodeLiveness updates the liveness record (e.g. during
heartbeats), it first does a noop sync write to all disks. This ensures
that a node with a stalled disk will fail to maintain liveness and lose
its leases.

However, this sync write could block indefinitely, and would not respect
the caller's context, which could cause the caller to stall rather than
time out. This in turn could lead to stalls higher up in the stack,
in particular with lease acquisitions that do a synchronous heartbeat.

This patch does the sync write in a separate goroutine in order to
respect the caller's context. The write operation itself will not
(can not) respect the context, and may thus leak a goroutine. However,
concurrent sync writes will coalesce onto an in-flight write.

Additionally, this runs the sync writes in parallel across all disks,
since we can now trivially do so. This may be advantageous on nodes with
many stores, to avoid spurious heartbeat failures under load.

Touches #81100.

Release note (bug fix): Disk write probes during node liveness
heartbeats will no longer get stuck on stalled disks, instead returning
an error once the operation times out. Additionally, disk probes now run
in parallel on nodes with multiple stores.

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Release justification: cluster availability improvement.

[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?

[cockroach-teamcity]

This change is