[crac] RFR: Active cleanup by CleanerImpl on checkpoint [v5]

[Radim Vansa]

> Rather than registering PhantomCleanableRef as a resource that gets cleaned up in the C/R thread force cleanup in the CleanerImpl as this already keeps a list of all eligible references.

Radim Vansa has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains six commits:

 - Merge branch 'crac' into sync_cleaner
 - Cleaner thread does not need to wait for checkpoint thread
 - Don't block the cleaner thread during C/R at all.
 - Clean GC'ed references on checkpoint
 - Merge branch 'crac' into sync_cleaner
 - Prevent concurrent cleanup by cleaner thread and checkpoint notifications

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Changes: https://git.openjdk.org/crac/pull/73/files
 Webrev: https://webrevs.openjdk.org/?repo=crac&pr=73&range=04
  Stats: 143 lines in 9 files changed: 99 ins; 33 del; 11 mod
  Patch: https://git.openjdk.org/crac/pull/73.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/73/head:pull/73

PR: https://git.openjdk.org/crac/pull/73