[crac] RFR: Ensure all notifications finish even if only daemon threads remain

[Anton Kozlov]

If as a result of beforeCheckpoint() no more non-daemon threads remain, it's possible that VM exits prematurely, before one of afterRestore() get a chance to create another non-daemon thread that will keep VM alive. Triggering checkpoint via jcmd (so checkpointRestore() method is executed on daemon attach-listener thread), increases probability to step in the problem.

The change ensures all notifications are done while there is at least one non-daemon thread. The notification methods are still called from the original thread.

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Commit messages:
 - Fix copyright
 - Notify on the original thread
 - Ensure all notifications finish even if only daemon threads remain

Changes: https://git.openjdk.org/crac/pull/62/files
 Webrev: https://webrevs.openjdk.org/?repo=crac&pr=62&range=00
  Stats: 146 lines in 2 files changed: 136 ins; 3 del; 7 mod
  Patch: https://git.openjdk.org/crac/pull/62.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/62/head:pull/62

PR: https://git.openjdk.org/crac/pull/62