RFR: bot: add liveness handler

[Erik Joelsson]

On Wed, 12 May 2021 19:35:16 GMT, Erik Helin <ehelin at openjdk.org> wrote:

> 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

Marked as reviewed by erikj (Reviewer).

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