RFR: Use a separate watchdog thread for the WorkItem scheduler

[Robin Westberg]

Use a separate watchdog thread to be able to detect if our ForkJoinPool stops executing tasks (which we've seen happen a few times). Not much to do in that case, just let the process restart.

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Commit messages:
 - Use a separate watchdog thread

Changes: https://git.openjdk.java.net/skara/pull/894/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=894&range=00
  Stats: 63 lines in 2 files changed: 61 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/894.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/894/head:pull/894

PR: https://git.openjdk.java.net/skara/pull/894