Geocoder improvements

[danvk]

• Purge unused files from geocache (will be more effective after Drop extended-grid and milstein coders #165)
• Delete old, failed geocodes, i.e. searches for intersections that do not return an intersection.
• Use the autocomplete API for geocoding.

The autocomplete API is recommended for "ambiguous" inputs. You can restrict to a geographic area and a match type (e.g. intersections). It gives you a place_id. To get a lat/lng, you have to run the place_id through the geocoder. This will require more requests, but should give more stable results.

The autocomplete API isn't that much of a win. Switching to OSM is going to be a better path forward.