release-21.2: backupccl: breakup the txn that inserts stats during cluster restore

[adityamaru]

Backport 1/1 commits from #75969.

/cc @cockroachdb/release

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We have seen instances of restores with hundreds of tables getting
stuck on inserting the backed up table stats into the system.table_stats
table on the restoring cluster. Previously, we would issue insert
statements for each table stat row in a single, long-running txn. If this
txn were to be retried a few times, we would observe intent buildup
on the system.table_stats ranges. Once these intents exceeded the
max_intent_bytes on the cluster, every subsequent txn retry would fall
back to the much more expensive ranged intent resolution. The only
remedy at this point would be to delete the BACKUP-STATISTICS file from
the bucket where the backup resides, and restore the tables with no
stats, relying on the AUTO STATS job to rebuild them gradually.

This change "batches" the insertion of the table stats to prevent the
above situation.

Fixes: #69207

Release note: None

Release justification: low risk, high impact change that fixes a class of stuck RESTOREs.

[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