Avoid large sql "in" clauses by operating on smaller batches of stacks
at a time.
To avoid transaction overhead and contention on the resource table,
the first deletions occur outside of a transaction (are
autocommitted). This is OK because the purge is re-rentrant -- we
won't lose any stack_id's to delete if something goes wrong before the
conn.begin() block. That is, we will not orphan any rows if the purge
is run multiple times where an error occurs.
Change-Id: I9edf0558ed54820842193560e323df6501411d1d
882a640f18