Keeping track of a lower bound of stream ID where we've deleted everything below makes the queries much faster. Otherwise, every time we scan for rows to delete we'd re-scan across all the rows that have previously deleted (until the next table VACUUM).
Using the new `TaskScheduler` meant that we'ed create lots of new
metrics (due to adding task ID to the desc of background process),
resulting in requests for metrics taking an increasing amount of CPU.
