WorkQueue: PointerWQ drain no longer waits for other queues

WorkQueue: PointerWQ drain no longer waits for other queues

If another (independent) queue was processing, drain could
block waiting.  Instead, allow drain to exit quickly if
no items are being processed and the queue is empty for
the current WQ.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit b118d7df1e34387b6e5649a5b205cf061598d0d4)