RFR: 8245203/8245204/8245208: ZGC: Don't hold the ZPageAllocator lock while committing/uncommitting memory

[Per Liden]

Hi,

Here are the latest patches for this. Contains more review comments from 
Stefan, and they have been rebased on today's jdk/jdk.

* 8246220: ZGC: Introduce ZUnmapper to asynchronous unmap pages
http://cr.openjdk.java.net/~pliden/8246220/webrev.2

* 8245208: ZGC: Don't hold the ZPageAllocator lock while 
committing/uncommitting memory
http://cr.openjdk.java.net/~pliden/8245208/webrev.3

* 8246265: ZGC: Introduce ZConditionLock
http://cr.openjdk.java.net/~pliden/8246265/webrev.0/

* (already review) 8245204: ZGC: Introduce ZListRemoveIterator
http://cr.openjdk.java.net/~pliden/8245204/webrev.0/

* (already review) 8245203: ZGC: Don't track size in ZPhysicalMemoryBacking
http://cr.openjdk.java.net/~pliden/8245203/webrev.0/


And for convenience, here's an all-in-one patch:
http://cr.openjdk.java.net/~pliden/zgc/commit_uncommit/all/webrev.1

cheers,
Per