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.

autopilot/graph.go

@@ -158,7 +158,7 @@ func (d *databaseChannelGraph) addRandChannel(node1, node2 *btcec.PublicKey,
 				return nil, err
 			}
 
-			dbNode, err := d.db.FetchLightningNode(vertex)
+			dbNode, err := d.db.FetchLightningNode(nil, vertex)
 			switch {
 			case err == channeldb.ErrGraphNodeNotFound:
 				fallthrough