RFR: 8220569: ZGC: Rename and rework ZUnmapBadViews to ZVerifyViews

[Per Liden]

The ZUnmapBadViews option is really a verification option, and should 
follow the ZVerify* naming convention. Further, the mechanism itself 
needs to be reworked a bit to not depend on pages being in the page 
table when cached, in preparation for later patches that reworks the 
ZPage life cycle.

Bug: https://bugs.openjdk.java.net/browse/JDK-8220569
Webrev: http://cr.openjdk.java.net/~pliden/8220569/webrev.0

/Per