rpc, p2p/simulations: use github.com/gorilla/websocket (#20289)

* rpc: improve codec abstraction

rpc.ServerCodec is an opaque interface. There was only one way to get a
codec using existing APIs: rpc.NewJSONCodec. This change exports
newCodec (as NewFuncCodec) and NewJSONCodec (as NewCodec). It also makes
all codec methods non-public to avoid showing internals in godoc.

While here, remove codec options in tests because they are not
supported anymore.

* p2p/simulations: use github.com/gorilla/websocket

This package was the last remaining user of golang.org/x/net/websocket.
Migrating to the new library wasn't straightforward because it is no
longer possible to treat WebSocket connections as a net.Conn.

* vendor: delete golang.org/x/net/websocket

* rpc: fix godoc comments and run gofmt

rpc/websocket.go

@@ -63,7 +63,7 @@ func (s *Server) WebsocketHandler(allowedOrigins []string) http.Handler {
 			return
 		}
 		codec := newWebsocketCodec(conn)
-		s.ServeCodec(codec, OptionMethodInvocation|OptionSubscriptions)
+		s.ServeCodec(codec, 0)
 	})
 }
 
@@ -171,5 +171,5 @@ func wsClientHeaders(endpoint, origin string) (string, http.Header, error) {
 
 func newWebsocketCodec(conn *websocket.Conn) ServerCodec {
 	conn.SetReadLimit(maxRequestContentLength)
-	return newCodec(conn, conn.WriteJSON, conn.ReadJSON)
+	return NewFuncCodec(conn, conn.WriteJSON, conn.ReadJSON)
 }