[crac] RFR: Active cleanup by CleanerImpl on checkpoint [v4]

[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 incrementally with one additional commit since the last revision:

  Cleaner thread does not need to wait for checkpoint thread

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Changes:
  - all: https://git.openjdk.org/crac/pull/73/files
  - new: https://git.openjdk.org/crac/pull/73/files/8ec3aee5..2b947915

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=crac&pr=73&range=03
 - incr: https://webrevs.openjdk.org/?repo=crac&pr=73&range=02-03

  Stats: 20 lines in 1 file changed: 1 ins; 8 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