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?)

lntest/itest/lnd_test.go

@@ -14248,12 +14248,12 @@ func testHoldInvoicePersistence(net *lntest.NetworkHarness, t *harnessTest) {
 	}
 
 	// Let Alice initiate payments for all the created invoices.
-	var paymentStreams []routerrpc.Router_SendPaymentClient
+	var paymentStreams []routerrpc.Router_SendPaymentV2Client
 	for _, payReq := range payReqs {
 		ctx, cancel := context.WithCancel(ctxb)
 		defer cancel()
 
-		payStream, err := net.Alice.RouterClient.SendPayment(
+		payStream, err := net.Alice.RouterClient.SendPaymentV2(
 			ctx, &routerrpc.SendPaymentRequest{
 				PaymentRequest: payReq,
 				TimeoutSeconds: 60,
@@ -14365,7 +14365,7 @@ func testHoldInvoicePersistence(net *lntest.NetworkHarness, t *harnessTest) {
 		ctx, cancel := context.WithCancel(ctxb)
 		defer cancel()
 
-		payStream, err := net.Alice.RouterClient.TrackPayment(
+		payStream, err := net.Alice.RouterClient.TrackPaymentV2(
 			ctx, &routerrpc.TrackPaymentRequest{
 				PaymentHash: hash[:],
 			},
@@ -14724,7 +14724,7 @@ func sendAndAssertSuccess(t *harnessTest, node *lntest.HarnessNode,
 	ctx, cancel := context.WithTimeout(context.Background(), defaultTimeout)
 	defer cancel()
 
-	stream, err := node.RouterClient.SendPayment(ctx, req)
+	stream, err := node.RouterClient.SendPaymentV2(ctx, req)
 	if err != nil {
 		t.Fatalf("unable to send payment: %v", err)
 	}
@@ -14742,7 +14742,7 @@ func sendAndAssertSuccess(t *harnessTest, node *lntest.HarnessNode,
 }
 
 // getPaymentResult reads a final result from the stream and returns it.
-func getPaymentResult(stream routerrpc.Router_SendPaymentClient) (
+func getPaymentResult(stream routerrpc.Router_SendPaymentV2Client) (
 	*lnrpc.Payment, error) {
 
 	for {