[15] RFR 8228818: Shenandoah: Processing weak roots in concurrent phase when possible

[Zhengyu Gu]

Please review this enhancement that moves some of weak root processing 
into concurrent phase whenever possible.

When concurrent class unloading is enabled, the weak roots that backed 
by OopStorage can be processed in concurrent phase, as Shenandoah native 
LRB can properly resolve the object and hide dead oops from mutators.


Bug: https://bugs.openjdk.java.net/browse/JDK-8228818
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8228818/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release)
   on x86_64 and x86_32 Linux
   specjvm on x86_64 Linux

Thanks,

-Zhengyu