graph/db+sqldb: implement ForEachSourceNodeChannel

In this commit, the ForEachSourceNodeChannel implementation of the
SQLStore is added. Since this is the first method of the SQLStore that
fetches channel and policy info, it also adds all the helpers that are
required to do so. These will be re-used in upcoming commits as more
"For"-type methods are added.

With this implementation, we convert the `TestForEachSourceNodeChannel`
such that it is run against SQL backends.

graph/db/graph_test.go

@@ -1183,7 +1183,7 @@ func TestAddEdgeProof(t *testing.T) {
 func TestForEachSourceNodeChannel(t *testing.T) {
 	t.Parallel()
 
-	graph := MakeTestGraph(t)
+	graph := MakeTestGraphNew(t)
 
 	// Create a source node (A) and set it as such in the DB.
 	nodeA := createTestVertex(t)