I managed to create a visitor now, however, instead of speeding things
up dijkstra_shortest_paths now runs several orders of magnitudes slower!
I created the visitor according to the bfs_name_printer example in the
BGL User Guide and Reference Book page 11.
The code seems to do what I want, all dists are either 0,1,2,3 or

numeric_limits<unsigned>::max(). But for a graph with 15000 edges it
takes a few ms to run dijkstra without a visitor but with it it takes 14
seconds. What am I doing wrong?