During 'NEOCluster.stop', there's often 1 'Serialized.background()' call too
many, and when it removed the last lock of the queue, another thread would hang
if was sleeping between 'q.put(lock)' and 'q.get().release()'.

An easy way to reproduce the bug was to add:
    if cls.pending is None:
        time.sleep(.01)
just before 'q.get().release()'