Currently, rows in system.commitlog_cleanups are only dropped on node restart,
so the table can accumulate an unbounded number of records.
This probably isn't a problem in practice, because tablet cleanups aren't that
frequent, but this patch adds a countermeasure anyway.
This patch makes the choice to delete the unneeded records right when new records
are added. This isn't ideal -- it would be more natural if the unneeded records
were deleted as soon as they become unneeded -- but it does the job with a
minimal amount of code.
f458a1bf3e