Kraken: move building of direct path

We may change the arrival path finder, so we need to do it after
building  paths
Kraken: fix a bug to really start the dijkstra

Add a function start_dijkstra_distance to start the dijkstra without the
need of a proximity list
Kraken: compute direct_pathes even when no stop_points are found
Jormungandr: Keep only one car journey