Improve preparation time

[easbar]

Some of the ideas taken from this discussion: https://github.com/Stunkymonkey/osm_ch/issues/1

• [x] cancel witness searches after a certain amount of nodes have been explored -> see #37
• [ ] use contracted_neighbors heuristic
• [ ] use flat array for preparation graph
• [ ] parallelization https://github.com/Stunkymonkey/osm_ch/issues/1#issuecomment-877780244
• [ ] contract only a certain fraction of nodes
• [x] find out why abstreet graph preparation is slow compared to e.g. plain OSM maps https://github.com/Stunkymonkey/osm_ch/issues/1#issuecomment-876583423 -> see #37
• [ ] clean up query code? https://github.com/Stunkymonkey/osm_ch/issues/1#issuecomment-877780244