backupccl: breakup the txn that inserts stats during cluster restore

[adityamaru]

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

[cockroach-teamcity]

This change is 

[adityamaru]

I can add some checkpointing to make sure we only insert table stats once, but that would involve a proto change. I wanted to get yall temperature on backporting this form of the fix.

[adityamaru]

Hmmm for a second I thought that breaking up the txn means that we could have a PK collision if the job were to be re-resumed. The stats table has a PK on statsID, tableID, but fortunately, the InsertNewStat method does not insert the backed up statsID, and relies on its default unique_row_id. So this change is still safe.

[stevendanna]

Overall it looks good to me.

[adityamaru]

TFTR!

bors r=stevendanna

[craig]

Build succeeded:

• GitHub CI (Cockroach)

[adityamaru]

blathers backport 21.2

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from 568c463 to blathers/backport-release-21.2-75969: POST https://api.github.com/repos/cockroachlabs/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 21.2 failed. See errors above.

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