Use the httputil.NewSingleHostReverseProxy instead of yhat/wsutil for … (#1348)

* Use the httputil.NewSingleHostReverseProxy instad of yhat/wsutil for websocket proxying. This correctly handles 404 responses with keep-alive by terminating the tunnel rather than keeping it alive

* Tidy up dependencies - yhat/wsutil is no longer required

* Update changelog to include reference to 1348

Co-authored-by: Matt Lilley <matt.lilley@securitease.com>
Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>