RFR 8223774: Shenandoah: Refactor ShenandoahRootProcessor and family

[Zhengyu Gu]

I would like to refactor ShenandoahRootProcessor and family, because it 
is bit hard to track and reason the combinations of incoming parameters, 
and combinations are likely to increase when moving more processing to 
concurrent phases.

Also, fixed a traversal bug that did not do 
StringDedup::gc_prologue()/gc_epilogue() in 
ShenandoahInitTraversalCollectionTask, bug showed up with refactored code.

The refacoring removed some verification assertions, but we now have 
equivalent functionality in verifier.

Bug: https://bugs.openjdk.java.net/browse/JDK-8223774
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8223774/webrev.00/index.html

Test:
   hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu