routing: rename routing Graph method

In preparation for having the ChannelGraph directly implement the
`routing.Graph` interface, we rename the `ForEachNodeChannel` method to
`ForEachNodeDirectedChannel` since the ChannelGraph already uses the
`ForEachNodeChannel` name and the new name is more appropriate since the
ChannelGraph currently has a `ForEachNodeDirectedChannelTx` method which
passes the same DirectedChannel type to the given call-back.

routing/bandwidth.go

@@ -63,7 +63,7 @@ func newBandwidthManager(graph Graph, sourceNode route.Vertex,
 
 	// First, we'll collect the set of outbound edges from the target
 	// source node and add them to our bandwidth manager's map of channels.
-	err := graph.ForEachNodeChannel(sourceNode,
+	err := graph.ForEachNodeDirectedChannel(sourceNode,
 		func(channel *graphdb.DirectedChannel) error {
 			shortID := lnwire.NewShortChanIDFromInt(
 				channel.ChannelID,