RFR 8227866: Shenandoah: Split weak root processing and class unloading in parallel cleaning task

[Zhengyu Gu]

Currently, Shenandoah uses shared ParallelCleaningTask to perform STW 
cleaning task, the cleaning task performs class unloading along with 
string deduplication cleaning, if it is enabled. This combination is 
undesirable, cause we want to have an option to not perform class 
unloading during STW.

This refactor splits cleaning task into two tasks, one performs class 
unloading and the other performs weak root cleaning up, so it is easy to 
enable/disable STW class unloading in later changes.

Bug: https://bugs.openjdk.java.net/browse/JDK-8227866
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8227866/webrev.00/index.html

Test:

   hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu