Flat index mode

[tmcw]

This will be a mode that disables merging by modifying min and max level constants to be the same, and replacing range queries with direct GET queries. This will test out @DennisOSRM's idea that avoiding range queries will be faster and simpler than trying to use ranges.

[DennisOSRM]

👍

[tmcw]

This is implemented in flat: https://github.com/mapbox/cardboard/tree/flat and used in https://github.com/mapbox/cardboard/tree/s3