RFR(S) 8206467: Refactor G1ParallelCleaningTask into shared

[Zhengyu Gu]

Hi,

Shenandoah has a similar version that was derived from 
G1ParallelCleaningTask, with additional time tracking of each cleaning task.

Due to code movement and renaming, the changeset appears to be large, 
but it is really not. Other than the renaming, followings are the actual 
diffs vs. current G1ParallelCleaningTask:

- G1StringDedupUnlinkOrOopsDoClosure is passed as a parameter.

- Counters (_string_processed, _string_removed, and etc.) in 
StringAndSymbolCleaningTask are volatile now, cause they are updated 
using atomic operations.

- Added ParallelCleaningTimes and ParallelCleaningTaskTimer classes for 
tracking times.

- ParallelCleaningTask::work() added time tracking code.


Bug: https://bugs.openjdk.java.net/browse/JDK-8206467
Webrev: http://cr.openjdk.java.net/~zgu/8206467/webrev.00/


Test:
   hotspot_gc on Linux 64 (fastdebug and release)


Thanks,

-Zhengyu