multi: let FetchLightningNode take an optional tx

In preparation for the next commit which will remove the
`*LightningNode` from the `ChannelEdgePolicy` struct,
`FetchLightningNode` is modified to take in an optional transaction so
that it can be utilised in places where a transaction exists.

routing/router.go

@@ -2802,7 +2802,7 @@ func (r *ChannelRouter) GetChannelByID(chanID lnwire.ShortChannelID) (
 func (r *ChannelRouter) FetchLightningNode(
 	node route.Vertex) (*channeldb.LightningNode, error) {
 
-	return r.cfg.Graph.FetchLightningNode(node)
+	return r.cfg.Graph.FetchLightningNode(nil, node)
 }
 
 // ForEachNode is used to iterate over every node in router topology.