8209841: [REDO] Refactor G1ParallelCleaningTask into shared

[Roman Kennke]

Looks good to me!
Thanks,
Roman

> 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