• GC dropped indexes.
• Disable the new schema changer for computed columns. See sql: enable adding columns which are not written to the current primary index #59149.
• Ensure that only one "new" schema change can be running on a table at a time, by restarting transactions that would initiate new schema changes (either from the old or the new schema changer) until there are no preexisting mutations on the table.
• Support reverting schema changes on failure or job cancelation.
• Properly checkpoint and read job progress for the index backfill.

Epic: CRDB-2356

Jira issue: CRDB-3234