routerrpc+lncli: add flag to suppress in-flight updates

This is a preparation for enabling the REST interface on routerrpc.
It provides REST clients that don't support server-side streaming
via keep-alive connections to use the streaming endpoint in the
typical request/response pattern. The url just needs to contain
?no_inflight_updates=true and only the terminal response is sent
back before the connection is closed.

cmd/lncli/cmd_pay.go

@@ -397,7 +397,7 @@ func sendPaymentRequest(ctx *cli.Context,
 
 	req.FeeLimitSat = feeLimit
 
-	showInflight := ctx.Bool(showInflightFlag.Name)
+	req.NoInflightUpdates = !ctx.Bool(showInflightFlag.Name)
 
 	stream, err := routerClient.SendPaymentV2(context.Background(), req)
 	if err != nil {
@@ -410,9 +410,9 @@ func sendPaymentRequest(ctx *cli.Context,
 			return err
 		}
 
-		if status.Status != lnrpc.Payment_IN_FLIGHT {
-			printRespJSON(status)
+		printRespJSON(status)
 
+		if status.Status != lnrpc.Payment_IN_FLIGHT {
 			// If we get a payment error back, we pass an error up
 			// to main which eventually calls fatal() and returns
 			// with a non-zero exit code.
@@ -422,10 +422,6 @@ func sendPaymentRequest(ctx *cli.Context,
 
 			return nil
 		}
-
-		if showInflight {
-			printRespJSON(status)
-		}
 	}
 }