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.

rpcserver.go

@@ -6135,7 +6135,7 @@ func (r *rpcServer) GetNodeInfo(ctx context.Context,
 	// With the public key decoded, attempt to fetch the node corresponding
 	// to this public key. If the node cannot be found, then an error will
 	// be returned.
-	node, err := graph.FetchLightningNode(pubKey)
+	node, err := graph.FetchLightningNode(nil, pubKey)
 	switch {
 	case err == channeldb.ErrGraphNodeNotFound:
 		return nil, status.Error(codes.NotFound, err.Error())
@@ -7107,7 +7107,7 @@ func (r *rpcServer) ForwardingHistory(ctx context.Context,
 			return "", err
 		}
 
-		peer, err := r.server.graphDB.FetchLightningNode(vertex)
+		peer, err := r.server.graphDB.FetchLightningNode(nil, vertex)
 		if err != nil {
 			return "", err
 		}