Consider acceleration profiles for short trips

[danpat]

When you request routes between two very close locations, we currently assume you'll be travelling at full speed all the way.

This leads us to under-estimate travel times for very short trips. Imagine feeding a mix of close-by coordinates, and medium-distancey coordinates into a /table API call and then sending the results to an route optimizer - the results can be less than ideal, depending on the spread.

For car routing, we should consider the effects of acceleration/deceleration. For longer journeys the effect is minimal, but for short trips, it can dominate the travel time. I think we need two things:

1. A way to specify the acceleration/braking curves either as a URL parameter, or as part of the car.lua profile
2. A way to decide whether all waypoints should be subject to the acceleration/braking penalties (e.g. waypoints where you stop vs ones where you don't)

We should also consider how this works with all plugins - i.e. I don't think acceleration should be considered for map matching, by default.

As a secondary feature, we could also consider acceleration when calculating turn costs.