Fixes a race on the dirty card queue completed buffer list between worker thread(s) performing a flush of a deferred store barrier (enqueueing a newly completed buffer) and worker thread(s) in the RSet updating code claiming completed buffers. Removed the routine that removes elements from the completed update buffer queue using a CAS.
Reviewed-by: ysr, tonyp
6720334: G1: don't update RSets of collection set regions during an evacuation pause
Introduced a deferred update mechanism for delaying the rset updates during the collection pause
Reviewed-by: apetrusenko, tonyp