We should add a knob to allow for control of schema change speed in 19.1.

We should also use this knob to slow down schema changes in 19.1 to address issues like #34744, #36385, and others preemptively.

In 19.2, we can turn the knob back up after addressing CPU spike (#34744) and other stability concerns.

Without this knob, users can only turn off this implementation if they run into problems. Since we expect this implementation to be better than 2.1's overall, it would be a shame to need to direct users back to 2.1.

Epic CRDB-8816

Jira issue: CRDB-4499