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?)

lnrpc/routerrpc/router_server.go

@@ -48,7 +48,7 @@ var (
 
 	// macPermissions maps RPC calls to the permissions they require.
 	macPermissions = map[string][]bakery.Op{
-		"/routerrpc.Router/SendPayment": {{
+		"/routerrpc.Router/SendPaymentV2": {{
 			Entity: "offchain",
 			Action: "write",
 		}},
@@ -56,7 +56,7 @@ var (
 			Entity: "offchain",
 			Action: "write",
 		}},
-		"/routerrpc.Router/TrackPayment": {{
+		"/routerrpc.Router/TrackPaymentV2": {{
 			Entity: "offchain",
 			Action: "read",
 		}},
@@ -214,13 +214,13 @@ func (s *Server) RegisterWithRootServer(grpcServer *grpc.Server) error {
 	return nil
 }
 
-// SendPayment attempts to route a payment described by the passed
+// SendPaymentV2 attempts to route a payment described by the passed
 // PaymentRequest to the final destination. If we are unable to route the
 // payment, or cannot find a route that satisfies the constraints in the
 // PaymentRequest, then an error will be returned. Otherwise, the payment
 // pre-image, along with the final route will be returned.
-func (s *Server) SendPayment(req *SendPaymentRequest,
-	stream Router_SendPaymentServer) error {
+func (s *Server) SendPaymentV2(req *SendPaymentRequest,
+	stream Router_SendPaymentV2Server) error {
 
 	payment, err := s.cfg.RouterBackend.extractIntentFromSendRequest(req)
 	if err != nil {
@@ -419,10 +419,10 @@ func (s *Server) QueryProbability(ctx context.Context,
 	}, nil
 }
 
-// TrackPayment returns a stream of payment state updates. The stream is
+// TrackPaymentV2 returns a stream of payment state updates. The stream is
 // closed when the payment completes.
-func (s *Server) TrackPayment(request *TrackPaymentRequest,
-	stream Router_TrackPaymentServer) error {
+func (s *Server) TrackPaymentV2(request *TrackPaymentRequest,
+	stream Router_TrackPaymentV2Server) error {
 
 	paymentHash, err := lntypes.MakeHash(request.PaymentHash)
 	if err != nil {
@@ -436,7 +436,7 @@ func (s *Server) TrackPayment(request *TrackPaymentRequest,
 
 // trackPayment writes payment status updates to the provided stream.
 func (s *Server) trackPayment(paymentHash lntypes.Hash,
-	stream Router_TrackPaymentServer) error {
+	stream Router_TrackPaymentV2Server) error {
 
 	router := s.cfg.RouterBackend