8209841: [REDO] Refactor G1ParallelCleaningTask into shared

[Zhengyu Gu]

This is redo of JDK-8206467.

- Fixed merging issue in original patch.

- Based on Thomas comments, moved time tracking of individual cleaning task.

The difference from current G1 implementation:

- G1StringDedupUnlinkOrOopsDoClosure is passed to ParallelCleaningTask 
as a parameter.

- Moved initialization of _initial_string_table_size in 
StringCleaningTask to the initializer list (as Kim suggested)

- Passes NULL StringDedupUnlinkOrOopsDoClosure to indicate string 
duduplication is disabled, and removed process_string_dedup parameter
in StringCleaningTask constructor.


Bug: https://bugs.openjdk.java.net/browse/JDK-8209841
Webrev: http://cr.openjdk.java.net/~zgu/8209841/webrev.00/

Test:

   hotspot_gc Linux x64 (fastdebug and release)


Thanks,

-Zhengyu