Webhook server can block

[dgw]

We had the webhook server start yielding timeouts when GitHub sent payloads to it.

Unfortunately there was nothing useful in the bot's logs. However, we know that wsgiref is not particularly suited for production use, and there's probably a better ServerAdapter that we could use with Bottle to get things the built-in WSGI reference implementation doesn't offer (higher concurrency, control over and protection from bad requests that never terminate, etc.).