routerrpc: rename rpcs to SendPaymentV2 and TrackPaymentV2

The message in the response stream changed. Rename the calls themselves,
to prevent older applications from getting decode errors. Especially
troublesome is the case where the request is executed (send payment),
but the application can't read the outcome (payment sent or not?)

cmd/lncli/cmd_pay.go

@@ -399,7 +399,7 @@ func sendPaymentRequest(ctx *cli.Context,
 
 	showInflight := ctx.Bool(showInflightFlag.Name)
 
-	stream, err := routerClient.SendPayment(context.Background(), req)
+	stream, err := routerClient.SendPaymentV2(context.Background(), req)
 	if err != nil {
 		return err
 	}
@@ -462,7 +462,7 @@ func trackPayment(ctx *cli.Context) error {
 		PaymentHash: hash,
 	}
 
-	stream, err := client.TrackPayment(context.Background(), req)
+	stream, err := client.TrackPaymentV2(context.Background(), req)
 	if err != nil {
 		return err
 	}