Summary: Splitting out a g1 allocator class to simply specialized allocators which can associate each allocation with a given context.
Reviewed-by: mgerdin, brutisso

Summary: After JDK-8035400 there were several opportunities to fix the visibility of several members of the G1ParScanThreadState class.
Reviewed-by: brutisso, mgerdin

Summary: Extract the G1ParScanThreadState class from G1CollectedHeap.?pp into its own files.
Reviewed-by: brutisso, mgerdin