RFR: 8241141: Restructure humongous object allocation in G1

[Stefan Johansson]

Hi,

Please review this refactoring of the humongous allocation code in G1.

Issue: https://bugs.openjdk.java.net/browse/JDK-8241141
Webrev: http://cr.openjdk.java.net/~sjohanss/8241141/00/

Summary
The allocation code for humongous objects is scattered between 
G1CollectedHeap and the HeapRegionManager. This change moves the 
allocating regions part into the manager, while heap is still 
responsible for initializing the object. The code that finds contiguous 
regions in the heap has also been refactored a bit to simplify the 
logic. Now we make sure that we only search among available regions when 
not wanting to expand the heap.

Testing
Tier 1-5 in Mach5 and local stress testing.

Thanks,
Stefan