Resolve mongodb connection drop issues (I think).

By switching the mongo connections from the raw Mongo adapter to
Mongoose, this seems to fix NREL/api-umbrella#17.

Mongoose uses the same underlying mongo driver, so why this fixes it is
a bit confusing. My only theories are:

- It's related to how Mongoose connects and the keepalive setting.
  Mongoose doesn't use the newer MongoClient.connect approach, and on a
  quick glance, there are maybe some differences in how keepalive
  settings get passed with the new MongoClient.connect).
- Or it's Mongoose's query buffering that helps resolve this issue.
  While we don't need most of Mongoose's extra features, its ability to
  buffer queries when in a disconnected state might be what fixes this.

Assuming this ends up working, the retry logic in api_key_validator.js
should be revisited.