RFR(XS): Heap memory usage counting not longer needs to be atomic

[Zhengyu Gu]

After guarding memory allocation with heap lock, memory usage counting also under heap lock
or executed by VM thread during a safepoint.

http://cr.openjdk.java.net/~zgu/shenandoah/memory_counting/webrev.00/

Test:

specjvm derby with fastdebug.

Thanks,

-Zhengyu