RFR: 8232649: ZGC: Add callbacks to ZMemoryManager

[Stefan Karlsson]

Hi all,

Please review this patch to add callbacks to ZMemoryManager.

https://cr.openjdk.java.net/~stefank/8232649/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8232649

This allows users of ZMemoryManager to get callbacks when memory regions 
are inserted, removed, split, and coalesced. This is needed to support 
Windows' stricter requirements for placeholder reserved memory.

Thanks,
StefanK