RFR: 1049: Watchdog should change health status to unhealthy

[Erik Joelsson]

This patch adds a callback for the Watchdog, which is used to flip the isHealthy status to false when the watchdog triggers. This will help us to more quickly discover when a bot runner isn't working if System.exit() isn't working (which happened today).

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Commit messages:
 - SKARA-1049

Changes: https://git.openjdk.java.net/skara/pull/1163/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1163&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1049
  Stats: 8 lines in 2 files changed: 5 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/1163.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1163/head:pull/1163

PR: https://git.openjdk.java.net/skara/pull/1163