RFR 8231293: Shenandoah: Traversal fails JVMTI tests

[Zhengyu Gu]

The problem is caused by LRB trying to evacuate dead oops.
At this point, traversal is completed, therefore, no more evacuation 
should happen. We should complete the marking bitmap and arm native 
barrier to prevent from-space oops from leaking to system.

There are duplicated works among fixup roots and parallel cleaning, 
let's address them in DKJDK-8231324.

Bug: https://bugs.openjdk.java.net/browse/JDK-8231293
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8231293/webrev.00/index.html

Test:
   hotspot_gc_shenandoah (fastdebug and release)
   vmTestbase/nsk/jvmti (fastdebug and release) normal and traversal
   modes.

Thanks,

-Zhengyu