RFR: 8210881: ZGC: Introduce ZRootsIteratorClosure

[Per Liden]

Introduce ZRootsIteratorClosure to allow piggybacking per-thread 
operations when iterating over thread roots. Use this to update the 
thread local address bad mask, instead of having that logic buried 
inside the ZRootIterator, which should not have knowledge of this. This 
mechanism will later also be used to retire/resize/remap TLABs.

Bug: https://bugs.openjdk.java.net/browse/JDK-8210881
Webrev: http://cr.openjdk.java.net/~pliden/8210881/webrev.0

/Per