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/graph.go

@@ -12,9 +12,9 @@ import (
 // Graph is an abstract interface that provides information about nodes and
 // edges to pathfinding.
 type Graph interface {
-	// ForEachNodeChannel calls the callback for every channel of the given
-	// node.
-	ForEachNodeChannel(nodePub route.Vertex,
+	// ForEachNodeDirectedChannel calls the callback for every channel of
+	// the given node.
+	ForEachNodeDirectedChannel(nodePub route.Vertex,
 		cb func(channel *graphdb.DirectedChannel) error) error
 
 	// FetchNodeFeatures returns the features of the given node.