RFR: 8219633: ZGC: Rename ZPageSizeMin to ZGranuleSize

[Per Liden]

The size of the unit of memory managed by ZGC is currently called 
ZPageSizeMin. However, this constant is used in places where we're not 
necessarily dealing with a ZPage, so calling it ZPageSizeMin isn't quite 
right. Rename ZPageSizeMin to ZGranuleSize.

Bug: https://bugs.openjdk.java.net/browse/JDK-8219633
Webrev: http://cr.openjdk.java.net/~pliden/8219633/webrev.0

/Per