consider cache stop_times to improve performance

[carocad]

Original issue taken from #174

Right now we rely solely on the trips available at the requested DateTime to find out the necessary stop.times every time that we try to find a trip.

https://github.com/hiposfer/kamal/blob/master/src/hiposfer/kamal/services/routing/directions.clj#L207

However, having the trips means that we can also know the stop.times and the stops available.

I think this would speed up the routing since we are doing all of these computations anyway but without this level of preprocessing.

I am not sure how much would this speed things up since we dont have a gtfs benchmark. So for the time being this is rather a nice to have.

[carocad]

Something that I can think of right now is basically to fetch all stop_times that are available and put them into a sorted-set like https://github.com/clojure/data.avl

Then sort those based on departure_time or on stop to have very fast query time.

NOTE: since clojure uses shared data structures, only the overhead of maintaining the indexes of the sets is added

[carocad]

closed in the reference above