Track and kill background sweep after targeted is enabled

[tpetracca]

My understanding of sweep things:

• Targeted sweep only cleans out update/deletes that have occurred after targeted sweep is enabled
• We still run background sweep by default everywhere because we want it to clear out cruft from before targeted sweep was enabled
• After enabling targeted sweep, running background sweep completely/successfully once should clean out all old cruft and remove the need to ever run background sweep ever again*

FR: Actually track the above and disable background sweep on tables who have successfully [background] swept at least once since TS was enabled*

* I guess there are some edge cases here. Like if you transitions from CONSERVATIVE to THOROUGH you'd want to re-enable TS and let it do 1 run.

[sandorw]

I believe that we no longer update the priority tables for background sweep, meaning that once it finishes one final pass through all tables, it will never sweep them again. We could certainly do the final step here (although it's more work) to persist that fact and no longer spin up background sweep threads, but we're most of the way there already.

This won't prevent problems where there are large tables that have never been successfully fully swept - as is now, background sweep will always make one final pass through them (which is desirable in the abstract).