[performance] Refactor RouteProcessor's edge time generation

[kuanb]

Edge time generation is where most of runtime is spent. The below image demonstrates the breakdown of time by each route in the AC Transit GTFS feed:

Total runtime is ~10-11 seconds on my current machine. Using mp increases that to 11-12 seconds (this is bad). Inefficiencies are tied up in the execution of the generate_all_observed_edge_costs method within generate_route_costs.