RFR: bot: add readiness handler

[Erik Helin]

Hi all,

please review this patch that adds a "readiness" handler to the bot runner's HTTP server. A readiness handler can be used to automate deployments and are usually queried to see when a service is ready to do work. Typically listening on paths such a `/ready` the readiness can help automatic deployment software differentiate between the two states "started" and "ready to work". The "readiness" handler will return `200` when the bot runner is ready to work and `404` when it isn't.

Thanks,
Erik

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Commit messages:
 - bot: add readiness handler

Changes: https://git.openjdk.java.net/skara/pull/1154/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1154&range=00
  Stats: 67 lines in 3 files changed: 66 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/1154.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1154/head:pull/1154

PR: https://git.openjdk.java.net/skara/pull/1154