Request for review(S): 6841831 G1: assert(contains_reference(from),"We just added it!") fires

[Igor Veresov]

During parallel rset updating we have to make sure that the worker ids 
of the refinement threads do not intersect with the worker ids that can 
be claimed by the mutator threads.

Webrev: http://cr.openjdk.java.net/~iveresov/6841831/webrev.00/