8190426: Lazily initialize refinement threads with UseDynamicNumberOfGCThreads Reviewed-by: sangheki, sjohanss

8182050: assert(_whole_heap.contains(p)) failed: Attempt to access p out of bounds of card marking array's _whole_heap Summary: Ignore zero-sized MemRegions to invalidate in G1SATBCardTableLoggingModRefBS::invalidate() Reviewed-by: ehelin, sjohanss