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

[Thomas Schatzl]

Hi all,

  can I have reviews on this change that makes the evacuation failure remove forwards phase split the work across threads within regions?

This work is a continuation of PR#7047 from @Hamlin-Li and latest improvements from @albertnetymk in that thread. This is reflected in the first commit, I added a fair amount of changes to fix bugs and streamline the code.  

Testing: tier1-8 with G1EvacuationFailureALot enabled, tier1-5 as is

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Commit messages:
 - disable G1EvacuationFailureALot by default again
 - Remove unneeded clear metadata phase
 - Remove debug code
 - More refactoring
 - Initial cleanup
 - some refactoring, fix clearing of opt index in cset
 - fix test
 - some cleanup
 - Cleanup, phase names, fixes
 - fixes for single bitmap
 - ... and 1 more: https://git.openjdk.org/jdk/compare/f2f0cd86...6fc07344

Changes: https://git.openjdk.org/jdk/pull/9980/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9980&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8256265
  Stats: 498 lines in 22 files changed: 271 ins; 97 del; 130 mod
  Patch: https://git.openjdk.org/jdk/pull/9980.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9980/head:pull/9980

PR: https://git.openjdk.org/jdk/pull/9980