RFR: 8240679: ZGC GarbageCollectorMXBean reports inaccurate post GC heap size for ZHeap pool

[Per Liden]

This patch adjusts when the memory manager reports it's post GC heap 
usage. Before it was reported at "Relocation Start" time, but with this 
patch it's reported at "End of GC cycle" time.

Bug: https://bugs.openjdk.java.net/browse/JDK-8240679
Webrev: http://cr.openjdk.java.net/~pliden/8240679/webrev.0
Testing: Tier1-7 with ZGC

/Per