routing: remove sourceNode from routingGraph interface

In this commit, we further reduce the routingGraph interface and this
time we make it more node-agnostic so that it can be backed by any graph
and not one with a concept of "sourceNode".

routing/payment_session_test.go

@@ -115,7 +115,7 @@ func TestUpdateAdditionalEdge(t *testing.T) {
 
 	// Create the paymentsession.
 	session, err := newPaymentSession(
-		payment,
+		payment, route.Vertex{},
 		func(routingGraph) (bandwidthHints, error) {
 			return &mockBandwidthHints{}, nil
 		},
@@ -195,7 +195,7 @@ func TestRequestRoute(t *testing.T) {
 	}
 
 	session, err := newPaymentSession(
-		payment,
+		payment, route.Vertex{},
 		func(routingGraph) (bandwidthHints, error) {
 			return &mockBandwidthHints{}, nil
 		},
@@ -211,9 +211,9 @@ func TestRequestRoute(t *testing.T) {
 
 	// Override pathfinder with a mock.
 	session.pathFinder = func(_ *graphParams, r *RestrictParams,
-		_ *PathFindingConfig, _, _ route.Vertex, _ lnwire.MilliSatoshi,
-		_ float64, _ int32) ([]*unifiedEdge, float64,
-		error) {
+		_ *PathFindingConfig, _, _, _ route.Vertex,
+		_ lnwire.MilliSatoshi, _ float64, _ int32) ([]*unifiedEdge,
+		float64, error) {
 
 		// We expect find path to receive a cltv limit excluding the
 		// final cltv delta (including the block padding).