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/mock_graph_test.go

@@ -165,7 +165,7 @@ func (m *mockGraph) addChannel(id uint64, node1id, node2id byte,
 // forEachNodeChannel calls the callback for every channel of the given node.
 //
 // NOTE: Part of the Graph interface.
-func (m *mockGraph) ForEachNodeChannel(nodePub route.Vertex,
+func (m *mockGraph) ForEachNodeDirectedChannel(nodePub route.Vertex,
 	cb func(channel *graphdb.DirectedChannel) error) error {
 
 	// Look up the mock node.