Encode keys for records in mongoDB

[pld]

In the interests of saving space, we can encode the observation record keys instead of "[some column name]" we could use "1", etc.

This would also require mapping queries, to the coded keys, and mapping the coded keys to the column names on their way into a dframe (we can pass columns= in the dframe constructor).

This assumes we are not planning on moving to a column store (which seems to be true).

[pld]

A couple more things:

1. this is a change to the way we store data that will require migrating or clearing any current databases
2. in observations we can remove the reserved key dataset_observation_id, it fulfills the same function as dataset_id
3. we can tie all our batch* functions closer to the Observation model, we do not expect anyone else to use these (with the potential exclusion of batch_read)
4. any encoding/decoding should happen at the batch level, to isolate it from the rest of the system (feels like a completely independent mix-in could be in order)

WIP here, modilabs:master...modilabs:encode-492