release-21.1: sql: fix deadlock when updating backfill progress

[blathers-crl]

Backport 1/1 commits from #69040 on behalf of @ajwerner.

/cc @cockroachdb/release

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The root cause here is that we acquired the mutex inside the transaction which
also laid down intents. This was not a problem in earlier iterations of this
code because of the FOR UPDATE logic which would, generally, in theory, order
the transactions such that the first one to acquire the mutex would be the
first to lay down an intent, thus avoiding the deadlock by ordering the
acquisitions. That was changed in #68244, which removed the FOR UPDATE.

What we see now is that you have a transaction doing the progress update which
hits a restart but has laid down an intent. Then we have a transaction which
is doing a details update that starts and acquires the mutex but blocks on the
intent of the other transaction. That other transaction now is blocked on the
mutex and we have a deadlock.

The solution here is to not acquire the mutex inside these transactions.
Instead, the code copies out the relevant state prior to issuing the
transaction. The cost here should be pretty minimal and the staleness in
the fact of retries is the least of my concerns.

No release note because the code in #68244 has never been released.

Touches #68951, #68958.

Release note: None

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Release justification:

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues.
• 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.

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