RFR 8225550: Shenandoah: Prevent SH::object_iterate() call's side-effects

[Zhengyu Gu]

This is another prerequisite for concurrent root processing. It prevents 
heap iteration from touching objects, that are considered unreachable by 
current GC cycle. Otherwise, LRB may evacuate the objects, make them 
alive from GC point of view.

The solution is to use the same roots to seed the iteration as ongoing 
GC cycle.

Bug: https://bugs.openjdk.java.net/browse/JDK-8225550
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8225550/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu