RFR: 8256265 G1: Improve parallelism in regions that failed evacuation

[Hamlin Li]

Currently G1 assigns a thread per failed evacuated region. This can in effect serialize the whole process as often (particularly with region pinning) there is only one region to fix up.
Try to improve the parallelsim when walking over the regions by 

 - first, split a region into tasks;
 - then, process these task in parallel and load balance among GC threads;
 - last, necessary cleanup

The perf test based on lastest implementation + JDK-8277736 shows that:

 - when ParallelGCThreads=32, when G1EvacuationFailureALotCSetPercent <= 50, the parallelism bring more benefit than regression;
- when ParallelGCThreads=128, whatever G1EvacuationFailureALotCSetPercent is, the parallelism bring more benefit than regression;

For detailed perf test result, please check:

 - https://bugs.openjdk.java.net/secure/attachment/97227/parallel.evac.failure-threads.32.png
 - https://bugs.openjdk.java.net/secure/attachment/97228/parallel.evac.failure-threads.128.png
 
NOTE: load balance part of code is almost same as G1ParScanThreadState, if necessary and feasible, consider to refactor this part into a shared code base.

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Commit messages:
 - Adjust worker cost by a factor; initialize task queues set and terminator threads by active workers
 - Fix wrong merge
 - Merge with master
 - Remove and merge code of G1ParRemoveSelfForwardPtrsTask into RemoveSelfForwardPtrsTask
 - Fix crashes in ~G1GCParPhaseTimesTracker(), G1PreRemoveSelfForwardClosure::do_heap_region, G1CollectedHeap::par_iterate_regions_array()=>~StubRoutines::atomic entry points; Refine comments
 - Fix inconsistent length between task queues and terminator
 - Fix crash when heap verification; Fix compilation error; Refine comments
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/6627/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6627&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256265
  Stats: 1055 lines in 22 files changed: 698 ins; 280 del; 77 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6627.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6627/head:pull/6627

PR: https://git.openjdk.java.net/jdk/pull/6627