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/unified_edges.go

@@ -125,7 +125,7 @@ func (u *nodeEdgeUnifier) addGraphPolicies(g Graph) error {
 	}
 
 	// Iterate over all channels of the to node.
-	return g.ForEachNodeChannel(u.toNode, cb)
+	return g.ForEachNodeDirectedChannel(u.toNode, cb)
 }
 
 // unifiedEdge is the individual channel data that is kept inside an edgeUnifier