RFR(S) 8227927: Move ParallelCleaningTask back to G1

[Zhengyu Gu]

ParallelCleaningTask was moved from G1 to shared directory, so 
Shenandoah can use it. Now, Shenandoah no longer uses it, it can return 
to G1.

However, Shenandoah still uses some of sub-tasks defined in 
shared/parallelCleaning.hpp/cpp, therefore, only ParallelCleaningTask 
and JVMCICleaningTask are moved to G1.

Bug: https://bugs.openjdk.java.net/browse/JDK-8227927
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8227927/webrev.00/index.html

Test:
   hotspot_gc (fastdebug and release) on Linux x86_64.
   Submit tests in progress


Thanks,

-Zhengyu