p2p: Use JSServer for comms rather than GoJsConn (#888)

* p2p: Use JSServer for comms rather than GoJsConn

This has several benefits:
 - it fixes a bug whereby you could not transmit >4k bytes to/from JS/Go land.
 - it more clearly exposes the interface point between Go and JS: a single
   global function call.
 - it presents a nicer API shape than the previous `net.Conn`.
 - it doesn't needlessly 'stream' data which is already sitting in-memory.

This is currently only active for local CS API traffic, another PR will
add Federation P2P support.

* Typo

cmd/dendritejs/main.go

@@ -156,10 +156,10 @@ func main() {
 	// Expose the matrix APIs via fetch - for local traffic
 	go func() {
 		logrus.Info("Listening for service-worker fetch traffic")
-
-		listener := go_http_js_libp2p.NewFetchListener()
-		s := &http.Server{}
-		go s.Serve(listener)
+		s := JSServer{
+			Mux: http.DefaultServeMux,
+		}
+		s.ListenAndServe("fetch")
 	}()
 
 	// We want to block forever to let the fetch and libp2p handler serve the APIs