Use database 'listener' to speed up discovery (D.R.Y.!)

We've already discovered the listener for a particular domain in the past, so why are we re-performing discovery each time the server starts afresh?

Even if a user changed the subdomain of their channel server there's no update mechanism at present and they wouldn't get pushed updates.

In the future we could perform discovery and update listeners for everyone at the same domain, but for now this seems like a sensible inclusion.