(TK-110) Added idle timeout setting to proxy 

One of the implications of the `idle-timeout` that hadn't dawned on me earlier is that if you give it a negative value, I believe Jetty will treat the timeout as "infinite" / "unlimited" -- although references to this being the case through the source code seem sketchy.

If we were to support this use case, I'd much prefer to have this exposed from the API as a more specific value that the user would set, e.g., a string of "unlimited", than some negative number.  That said, I'm not inclined to even try to support this use case at present.

So that we don't end up with users trying to take advantage of undocumented behavior that would be hard to break later, I suggest for now that we add some validation here to reject a negative number as being schema invalid.  Sound okay?