Load derived tables through map-reduce

This allows us to make arbitarily large derived slices, which can be
further aggregated for final usage. This allows us to obtain large speed
benefits by pre-aggregating data for front-end users.

The code in this is totally wild, for what it's worth. I convert JSON to 
Clojure data structures, which I use to write Clojure code that generates 
JavaScript to send to MongoDB.