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.
Erik Johnston