Upgrade slick to 2.1.0

Also contains some starts to refactors. Making this PR now as to match up with the upgrade to slick 2.1.0 in GeoTrellis and the gtfs-parser project.

Feel free to ask questions about the work-in-progress refactor on this PR. I'm making comments to point out a couple of things that I changed.
This should fix the [Travis scala test error](https://travis-ci.org/WorldBank-Transport/open-transit-indicators/builds/34330047) `Unspecified value parameter session`, right?