graph/db+sqldb: impl ForEachNodeCached and ForEachChannel

Which let's us run `TestGraphTraversal` against our SQL backends.

graph/db/graph_test.go

@@ -1277,7 +1277,7 @@ func TestForEachSourceNodeChannel(t *testing.T) {
 func TestGraphTraversal(t *testing.T) {
 	t.Parallel()
 
-	graph := MakeTestGraph(t)
+	graph := MakeTestGraphNew(t)
 
 	// We'd like to test some of the graph traversal capabilities within
 	// the DB, so we'll create a series of fake nodes to insert into the