Add support for pushing data to udp services.

Because HAProxy does not support udp, an alternative is required. This commit
adds an optional "udp_forwarding" section to the service level configuration.
The only configuration is a port, over which udp data will be forwarded to one
of the service's host:port combinations. Both "udp_forwarding" and "haproxy" cannot be present.

Some example use cases of this would be forwarding data to an external logging service or external monitoring services.

This branch does not support incoming udp data but that felt like a much rare use case to me.