RFR: 8147814: Move allocation and verification code out of g1collectedheap

[David Lindholm]

Hi.

As a first step in the technical debt task of splitting the enormous 
g1CollectedHeap class into smaller units I have moved allocation code to 
the g1Allocator class, and created a new class called g1HeapVerifier and 
moved all verification code there.

Could I please have a couple of reviews for this patch?

Bug: https://bugs.openjdk.java.net/browse/JDK-8147814
Webrev: http://cr.openjdk.java.net/~david/JDK-8147814/webrev.00/


Thanks,
David