routing: define graphParams and restrictParams

To avoid the findPath parameter list getting out of hand, we define new
structs that wraps the mandatory and optional parameters to findPath.

routing/router_test.go

@@ -1872,8 +1872,15 @@ func TestFindPathFeeWeighting(t *testing.T) {
 	// the edge weighting, we should select the direct path over the 2 hop
 	// path even though the direct path has a higher potential time lock.
 	path, err := findPath(
-		nil, ctx.graph, nil, sourceNode, target, ignoreVertex,
-		ignoreEdge, amt, noFeeLimit, nil,
+		&graphParams{
+			graph: ctx.graph,
+		},
+		&restrictParams{
+			ignoredNodes: ignoreVertex,
+			ignoredEdges: ignoreEdge,
+			feeLimit:     noFeeLimit,
+		},
+		sourceNode, target, amt,
 	)
 	if err != nil {
 		t.Fatalf("unable to find path: %v", err)