RFR 8226311: Shenandoah: Concurrent evacuation of OopStorage backed weak roots

[Zhengyu Gu]

Please review this patch that moves evacuation of OopStorage backed weak 
roots to concurrent phase, while JVMTI and JFR weak roots are still 
evacuated at pauses.

It is a followup of JDK-8225534 [1], where we put infrastructure for 
moving root evacuation to concurrent phase.

Bug: https://bugs.openjdk.java.net/browse/JDK-8226311
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8226311/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu

[1] https://bugs.openjdk.java.net/browse/JDK-8225534