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/server_test.go

@@ -77,7 +77,7 @@ func runTestScript(t *testing.T, file string) {
 
 	clientConn, serverConn := net.Pipe()
 	defer clientConn.Close()
-	go server.ServeCodec(NewJSONCodec(serverConn), OptionMethodInvocation|OptionSubscriptions)
+	go server.ServeCodec(NewCodec(serverConn), 0)
 	readbuf := bufio.NewReader(clientConn)
 	for _, line := range strings.Split(string(content), "\n") {
 		line = strings.TrimSpace(line)