Switch to knex.js

[prashtx]

Knex.js has a streaming interface that uses a cursor (via pg-query-stream). Since we're accessing chunks of fairly raw time-series data, rather than objects with more structured relationships, we don't take advantage of Sequelize's higher-level features. Initial investigations suggest that knex might just be nicer to work with, too.

The streaming interface indirectly necessitates the use of the javascript pg client (vs. the native client), so we should look at the performance before and after.