RFC: ZGC: Uncommit Unused Memory

[Per Liden]

Hi,

I prepared a new JEP [1], about adding support in ZGC for uncommitting 
unused heap memory. Implementing this is fairly straight forward, 
especially after the work to simplify the ZPage life cycle has been 
completed [2]. However, feedback is more than welcomed, especially on 
the policy part and command line options.

cheers,
Per

[1] https://bugs.openjdk.java.net/browse/JDK-8220347
[2] 
https://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2019-March/025076.html