graph/db: unexport methods that take a transaction

Unexport and rename the methods that were previously used by the
graphsession package.

graph/db/graph_test.go

@@ -3915,7 +3915,7 @@ func BenchmarkForEachChannel(b *testing.B) {
 	}
 }
 
-// TestGraphCacheForEachNodeChannel tests that the ForEachNodeDirectedChannelTx
+// TestGraphCacheForEachNodeChannel tests that the forEachNodeDirectedChannel
 // method works as expected, and is able to handle nil self edges.
 func TestGraphCacheForEachNodeChannel(t *testing.T) {
 	graph, err := MakeTestGraph(t)
@@ -3952,7 +3952,7 @@ func TestGraphCacheForEachNodeChannel(t *testing.T) {
 
 	getSingleChannel := func() *DirectedChannel {
 		var ch *DirectedChannel
-		err = graph.ForEachNodeDirectedChannelTx(nil, node1.PubKeyBytes,
+		err = graph.forEachNodeDirectedChannel(nil, node1.PubKeyBytes,
 			func(c *DirectedChannel) error {
 				require.Nil(t, ch)
 				ch = c