lnd+lnrpc: enable WebSocket ping/pong messages

Fixes #4497 by sending out ping messages in a regular interval to make
sure the connection is still alive.

lnd.go

@@ -1266,7 +1266,8 @@ func startRestProxy(cfg *Config, rpcServer *rpcServer, restDialOpts []grpc.DialO
 
 	// Wrap the default grpc-gateway handler with the WebSocket handler.
 	restHandler := lnrpc.NewWebSocketProxy(
-		mux, rpcsLog, lnrpc.LndClientStreamingURIs,
+		mux, rpcsLog, lnrpc.DefaultPingInterval, lnrpc.DefaultPongWait,
+		lnrpc.LndClientStreamingURIs,
 	)
 
 	// Use a WaitGroup so we can be sure the instructions on how to input the