RFR: 8205922: Add reference iteration mode that skips visiting the referents

[Stefan Karlsson]

Hi all,

Please review this patch to allow us to skip visiting the 
j.l.Reference::referent during oop_iteration.

http://cr.openjdk.java.net/~stefank/8205922/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8205922

One use-case for this is when ZGC does verification at the end of the 
Mark End pause. At that point we have complete marking information roots 
and fields are good, but the processing of j.l.Reference::referents are 
deferred to the concurrent phase after this pause. In this case we would 
like to skip applying the verification closure to the referents.

Thanks,
StefanK