[crac] RFR: Prevent concurrent cleanup by cleaner thread and checkpoint notifications

[Radim Vansa]

We block the cleaner thread to prevent race conditions between this thread and checkpointing thread invoking clean().
When the cleanup starts in cleaner thread the checkpoint will skip it, but without waiting for the cleanup to finish (which might be critical for the checkpoint, e.g. closing FDs).
The limitation is that code performing C/R must not wait on any task completed by the cleaner.

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Commit messages:
 - Prevent concurrent cleanup by cleaner thread and checkpoint notifications

Changes: https://git.openjdk.org/crac/pull/73/files
 Webrev: https://webrevs.openjdk.org/?repo=crac&pr=73&range=00
  Stats: 73 lines in 3 files changed: 64 ins; 7 del; 2 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