graph/db+sqldb: implement ChanUpdatesInHorizon

Add `ChanUpdatesInHorizon` method to the SQLStore. This lets us run
`TestChanUpdatesInHorizon` against our SQL backends.

graph/db/graph_test.go

@@ -1937,7 +1937,7 @@ func TestChanUpdatesInHorizon(t *testing.T) {
 	t.Parallel()
 	ctx := context.Background()
 
-	graph := MakeTestGraph(t)
+	graph := MakeTestGraphNew(t)
 
 	// If we issue an arbitrary query before any channel updates are
 	// inserted in the database, we should get zero results.