RFR: 8232649: ZGC: Add callbacks to ZMemoryManager

[Per Liden]

Looks good.

/Per

On 10/21/19 4:06 PM, Stefan Karlsson wrote:
> 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