RFR: bot: add liveness handler

[Erik Helin]

Hi all,

please review this patch that adds a liveness handler to the bot runner's HTTP server. The liveness handler, typically acting on the `/healthz` path, returns `200` if everything is well and `500` otherwise.

Thanks,
Erik

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Commit messages:
 - bot: add liveness handler

Changes: https://git.openjdk.java.net/skara/pull/1155/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1155&range=00
  Stats: 64 lines in 3 files changed: 63 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/1155.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1155/head:pull/1155

PR: https://git.openjdk.java.net/skara/pull/1155