release-23.2: sql/schemachanger: apply admission control to merge phase of backfill

[blathers-crl]

Backport 1/1 commits from #113713 on behalf of @fqazi.

/cc @cockroachdb/release

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Previously, when merging indexes as a part of our MVCC-compliant protocol, we did not properly flag the writes with the correct priority. As a result, on certain workloads, creating a secondary index could tank performance. To address this, this patch will set the priority for the writes during the merge phase of the backfill protocol.

Fixes: #113705

Release note (performance): Address performance regression that can
happen when the declarative schema changer is being used to create an
index with a concurrent workload.

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Release justification: high priority bug fix

[blathers-crl]

Thanks for opening a backport.

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[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

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[cockroach-teamcity]

This change is 

[sumeerbhola]

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @rafiss)