[crac] RFR: Ensure all notifications finish even if only daemon threads remain [v6]

[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.

Anton Kozlov has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains ten commits:

 - Fix test build
 - Merge remote-tracking branch 'jdk/crac/crac' into daemon-after-restore
 - Move KeepAlive to separate class, handle interrupts
 - Cleanup
 - Fix recursiveCheckpoint test
 - Test update
 - Fix copyright
 - Notify on the original thread
 - Ensure all notifications finish even if only daemon threads remain

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Changes: https://git.openjdk.org/crac/pull/62/files
 Webrev: https://webrevs.openjdk.org/?repo=crac&pr=62&range=05
  Stats: 203 lines in 4 files changed: 188 ins; 7 del; 8 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