Improve preparation time

[easbar]

Some of the ideas taken from this discussion: Stunkymonkey/osm_ch#1

• cancel witness searches after a certain amount of nodes have been explored -> see Add parameters to limit number of settled nodes during preparation, much faster preparation for certain graphs #37
• use contracted_neighbors heuristic
• use flat array for preparation graph
• parallelization Turning this into a general-purpose library Stunkymonkey/osm_ch#1 (comment)
• contract only a certain fraction of nodes
• find out why abstreet graph preparation is slow compared to e.g. plain OSM maps Turning this into a general-purpose library Stunkymonkey/osm_ch#1 (comment) -> see Add parameters to limit number of settled nodes during preparation, much faster preparation for certain graphs #37
• clean up query code? Turning this into a general-purpose library Stunkymonkey/osm_ch#1 (comment)