6608385: G1: need to support parallel reference processing Summary: Implement support for ParallelRefProcEnabled in the reference processing that takes place at the end of G1 concurrent marking. Reviewed-by: tonyp, ysr

7014261: G1: RSet-related failures Summary: A race between the concurrent cleanup thread and the VM thread while it is processing the "expanded sparse table list" causes both threads to try to free the same sparse table entry and either causes one of the threads to fail or leaves the entry in an inconsistent state. The solution is purge all entries on the expanded list that correspond go regions that are being cleaned up. Reviewed-by: brutisso, johnc