RFR: 8229017: ZGC: Various cleanups of ZVerify

[Per Liden]

Hi,

This patch does various cleanups of ZVerify, basically a post-commit 
review of JDK-8227175. The patch mostly moves some code around and 
adjusts a few names. However, there's also one bug fix and one logic change:

* ZVerify::roots_strong() didn't have a ZStatTimerDisable.

* The call to ClassLoaderDataGraph::clear_claimed_marks() was moved from 
ZMarkConcurrentRootsTask() to ZConcurrentRootsIterator(), and now only 
clears the claim type the iterator actually used (instead of all types).

Bug: https://bugs.openjdk.java.net/browse/JDK-8229017
Webrev: http://cr.openjdk.java.net/~pliden/8229017/webrev.0

/Per