RFR: Shortcutting RP during final mark misses JNI Weak Refs

[Aleksey Shipilev]

Turns out, there is landmine in RP code: we cannot shortcut on
has_discovered_references() at our final mark reference processing, because that
misses the JNI Weak Refs processing. Weak Refs are not counted towards
"discovered" refs now, so we can omit marking them on shortcut, breaking the heap.

Fix:
  http://cr.openjdk.java.net/~shade/shenandoah/refproc-shortcut-jniweak/webrev.01/

Heap verifier fails on jcstress because of this.

Thanks,
-Aleksey