graph/db+sqldb: impl PruneGraphNodes

Which lets us run `TestChannelEdgePruningUpdateIndexDeletion` against
our SQL backends.

graph/db/graph_test.go

@@ -3291,7 +3291,7 @@ func TestPruneGraphNodes(t *testing.T) {
 	t.Parallel()
 	ctx := context.Background()
 
-	graph := MakeTestGraph(t)
+	graph := MakeTestGraphNew(t)
 
 	// We'll start off by inserting our source node, to ensure that it's
 	// the only node left after we prune the graph.