The SATBMarkQueueSet usage model is pretty simple, and doesn't need a lot of the mechanism presently provided by PtrQueueSet (to also support G1DirtyCardQueueSet).

If we moved most of the existing PtrQueueSet to G1DirtyCardQueueSet, we could improve SATBMarkQueueSet, including using a LockFreeStack to manage the set of completed buffers, rather than a locked list, eliminating the need for the "access" ranked SATB_Q_CBL_mon.