DB_QUEUE tables.

close(): We can really simplify this and make it more robust for when
we add new tables, by relying on the fact that the base class
maintains its own list of opened tables, and the base class close()
method closes them in turn.

autopack(): Default the gc argument to False.

_collect_objs(), _mark(), _sweep(): Add an escape hatch for the pack
operation inside the inner loops of each of these methods.  That way,
we don't have to wait until the loops are finished to exit the pack
operation, if stop() has been requested by the main thread.

_Autopack: Use the _WorkThread base class.