Initial work on Analytics

* Add InfluxDB as a requirement for master
* Add `Master.Analytics` module, which stores `session_start` and `listen`
  events, and is then responsible for turning those events into sessions.
* Master: Load Analytics module (if configured), and attach its log listener
  to our logger
* FIXME: Need to fill out the logic for computing sessions after a restart
* TODO: No query logic has been done yet