RFR 8229213: Shenandoah: Allow VM global oop storage to be processed concurrently

[Zhengyu Gu]

JDK-8227653 introduced new VM global oop storage, and piggyback it to 
SystemDictionary for oop processing. This is not desirable for 
Shenandoah, since SystemDictionary is defined as a serial root, that can 
only be processed at paused.

This patch refactored oop storage backed roots and grouped jni handles 
and vm global oop storage into ShenandohVMRoots, that allows to be 
processed concurrently or at pauses, as currently 
ShenandoahJNIHandleRoots does.


Bug: https://bugs.openjdk.java.net/browse/JDK-8229213
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8229213/webrev.00/index.html

Test:
   hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu