RFR (S): 8185580: Refactor Threads::possibly_parallel_oops_do() to use Threads::parallel_java_threads_do()

[Roman Kennke]

This is a follow-up to my earlier safepoint parallel cleanup work.

Currently we have 2 places where we apply the parallel claiming protocol 
when iterating threads, that is Threads::parallel_java_threads_do() and 
Threads::possibly_parallel_oops_do(). In order to avoid code 
duplication, the latter should call the former, using a private 
ThreadClosure. We already had one bug (JDK-8185273) that was caused by 
an inconsistency between the two.

The only other user of parallel_java_threads_do() already filters out 
any non-Java-threads, which means that doing the extra processing of the 
VMThread should be ok. I renamed the method to parallel_threads_do() to 
reflect that it not only does the Java threads.

Webrev:
http://cr.openjdk.java.net/~rkennke/8185580/webrev.00/ 
<http://cr.openjdk.java.net/%7Erkennke/8185580/webrev.00/>
Bug:
https://bugs.openjdk.java.net/browse/JDK-8185580

Tested successfully by running hotspot_gc tests, which should cover most 
if not all uses of those methods.

Roman