Use parallel OopStorage iterator to scan JNI, JVMTI and JFR (strong&weak) roots

[Roman Kennke]

We currently process JNI handles and the weak roots serially. Those are
now backed by OopStorage and can easily be parallelized:

http://cr.openjdk.java.net/~rkennke/par-roots/webrev.00/

This should yield slightly better concurrency during root-marking.
(Can't do any useful testing on puny laptop, but can't see why it would
cause troubles either)

Testing: tier3_gc_shenandoah

Next step: this (at least the JNI and SystemDictionary/VM handles)
should be easy to do *concurrent* with Java threads by implementing the
*not_in_heap stuff in ShenandoahBarrierSet.

We should consider (later) that WeakProcessor is also called in our
weak-ref-processing to update remaining roots. It's probably doing
unnecessary double work, and this should probably also be parallelized.

Also, we need to consider (later) the string table roots. It seems that
this is always scanned/marked when we do ref-processing, but it seems
unrelated. I believe this should also go under the other weak roots and
only be visited for updating remaining refs. As it is now, it would keep
interned strings alive for no reason when ref-processing is active.

Roman