graph: temporarily rename some ChannelGraph methods

Add the `Tx` suffix to both ForEachNodeDirectedChannelTx and
FetchNodeFeatures temporarily so that we free up the original names for
other use. The renamed methods will be removed or unexported in an
upcoming commit. The aim is to have no exported methods on the
ChannelGraph that accept a kvdb.RTx as a parameter.

routing/integrated_routing_context_test.go

@@ -394,11 +394,11 @@ func (g *mockGraphSessionChanDB) close() error {
 func (g *mockGraphSessionChanDB) ForEachNodeChannel(nodePub route.Vertex,
 	cb func(channel *graphdb.DirectedChannel) error) error {
 
-	return g.graph.ForEachNodeDirectedChannel(g.tx, nodePub, cb)
+	return g.graph.ForEachNodeDirectedChannelTx(g.tx, nodePub, cb)
 }
 
 func (g *mockGraphSessionChanDB) FetchNodeFeatures(nodePub route.Vertex) (
 	*lnwire.FeatureVector, error) {
 
-	return g.graph.FetchNodeFeatures(g.tx, nodePub)
+	return g.graph.FetchNodeFeaturesTx(g.tx, nodePub)
 }