See #61386 (comment) and discussion in https://cockroachlabs.slack.com/archives/C2C5FKPPB/p1614790893188800.

Currently, each transaction issued by columnBackfiller.runChunk will perform a commit-wait stage of about 800ms when writing to a GLOBAL table. This adds up, as the ColumnBackfiller can issue many small transactions. The commit-wait step is meant to ensure linearizability with future reads of the values written by the column backfiller. However, there's no need to pay this cost on each chunk - we only need reads after the backfiller is completely finished to be guaranteed to observe its writes. So we should be able to pay the commit-wait cost once.

Proposal to lift commit-wait into the columnBackfiller:

1. add SkipCommitWait API to kv.Txn
2. call SkipCommitWait on txn in columnBackfiller.runChunk
3. record transaction commit time in columnBackfiller.runChunk
4. hlc.SleepUntil the maximum of all chunk commit timestamps in columnBackfiller.flush