release-24.1: fs,engineccl: allow reads to continue when writes are stalled

[sumeerbhola]

Backport 1/1 commits from #123057.

/cc @cockroachdb/release

---

Both fs.FileRegistry and engineccl.DataKeyManager held an internal mutex when updating their state, that included write IO to to update persistent state. This would block readers of the state, specifically file reads that need a file registry entry and data key for the file to successfully open and read a file.

Blocking these reads due to slow or stalled write IO is not desirable, since the read could succeed if the relevant data is in the page cache. Specifically, with the new WAL failover feature, we expect the store to keep functioning when disk writes are temporarily stalled, since the WAL can failover. This expectation is not met if essential reads block on non-essential writes that are stalled.

This PR changes the locking in the FileRegistry and DataKeyManager to prevent writes from interfering with concurrent reads.

Epic: none

Fixes: #98051
Fixes: #122364

Release note: None

Release justification: High priority fix to encryption-at-rest behavior under WAL failover, which allows reads to continue.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
• Backports should not break backwards-compatibility.
• Backports should change as little code as possible.
• Backports should not change on-disk formats or node communication protocols.
• Backports should not add new functionality (except as defined
  here).
• Backports must not add, edit, or otherwise modify cluster versions; or add version gates.
• All backports must be reviewed by the owning areas TL and one additional
  TL. For more information as to how that review should be conducted, please consult the backport
  policy.

If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• 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. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.
• Your backport must be accompanied by a post to the appropriate Slack
  channel (#db-backports-point-releases or #db-backports-XX-X-release) for awareness and discussion.

Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is 

[sumeerbhola]

Manual backport, to compensate for small interface change in master introduced in 91f53ea

[jbowens]

lgtm