RFR: 8247213: G1: Reduce usage of volatile in favour of Atomic operations

[Leo Korinth]

Hi, I have created a RFR in the hotspot-gc-dev mailing list that mostly 
changes stuff in G1. However I am also doing changes in atomic.hpp 
(mostly adding an AtomicValue<> class) so there might be interest 
outside gc. If you have interest, please follow that thread.

Thanks,
Leo