graph: let FetchNodeFeatures take an optional read tx

For consistency in the graphsessoin.graph interface, we let the
FetchNodeFeatures method take a read transaction just like the
ForEachNodeDirectedChannel. This is nice because then all calls in the
same pathfinding transaction use the same read transaction.

routing/integrated_routing_context_test.go

@@ -400,5 +400,5 @@ func (g *mockGraphSessionChanDB) ForEachNodeChannel(nodePub route.Vertex,
 func (g *mockGraphSessionChanDB) FetchNodeFeatures(nodePub route.Vertex) (
 	*lnwire.FeatureVector, error) {
 
-	return g.graph.FetchNodeFeatures(nodePub)
+	return g.graph.FetchNodeFeatures(g.tx, nodePub)
 }