release-21.2: sql: new SQL Stats cluster settings to improve write traffic

[blathers-crl]

Backport 1/1 commits from #78446 on behalf of @Azhng.

/cc @cockroachdb/release

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Resolves #78339

Previously, SQL Stats flushed to system table as soon as the in-memory
buffer is full. This means the size of the system tables that back SQL
Stats could grow faster than the cleanup job. Additionally, when the
SQL Stats flush is disabled, the SQL Stats is unable to collect any more
new statement / transaction statistics when the in-memory store is full.

This commit introduces two non-public cluster settings:

• sql.stats.flush.minimum_interval: this setting limits minimum interval
  between each flush operation. If a flush operation is triggered sooner
  than what is allowed by the minimum interval, (e.g. when the in-memory
  SQL Stats store is full), the flush operation is aborted.
• sql.stats.flush.force_cleanup.enabled: which allows the
  in-memory SQL Stats to be cleared at the interval specified by
  sql.stats.flush.interval, even if the SQL Stats flush is disabled.

This commit also updated the stmt_grouping_in_explicit_txn data driven
test to ensure the output order is deterministic.

Release note: None

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Release justification: low risk high benefit change

[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 

[matthewtodd]

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @Azhng and @maryliag)

[maryliag]

Reviewable status: complete! 1 of 0 LGTMs obtained (and 1 stale) (waiting on @Azhng)