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
An update buffer is not processed correctly, which causes roots into the collection set not to be scanned and, hence, for the heap to be corrupted. The cause is that an object is accessed after it has been explicitly deleted, which causes a race.
Reviewed-by: jcoomes, ysr
