Reviewer Mike - Further proxying support

This covers the two issues from story 327:

 * Bono should record route the Bono cluster rather than the individual Bono nodes when not acting as edge proxy.
 * Bono needs to correctly add the 'orig' parameter when forwarding requests upstream.

I've also taken the opportunity to move the target selection code from `proxy_calculate_targets` to `proxy_perform_edge_routing` and to allow the router to specify a target if it desires (similar to the AS handling code).