[crac] RFR: Ensure all notifications finish even if only daemon threads remain [v5]

[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 incrementally with one additional commit since the last revision:

  Move KeepAlive to separate class, handle interrupts

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Changes:
  - all: https://git.openjdk.org/crac/pull/62/files
  - new: https://git.openjdk.org/crac/pull/62/files/c2cfae5d..a3b1242e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=crac&pr=62&range=04
 - incr: https://webrevs.openjdk.org/?repo=crac&pr=62&range=03-04

  Stats: 132 lines in 3 files changed: 75 ins; 52 del; 5 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