RFR: 8057536: Refactor G1 to allow context specific allocations

[Stefan Johansson]

Hi,

Please review these changes for:
https://bugs.openjdk.java.net/browse/JDK-8057536

Webrev:
http://cr.openjdk.java.net/~sjohanss/8057536/webrev.00/

Summary:
These changes are made to allow G1 to do context specific allocation. As 
part of this a G1Allocator class has be refactored out of 
G1CollectedHeap which can be extended to implement the specific context 
code. Currently only the G1DefaultAllocator is implemented, this 
allocator only makes use of one context to have the same behavior as 
prior to this enhancement.

Testing:
* JPRT
* Manual verification
* Local ute runs of vm.quick.testlist

Thanks,
Stefan