Track processes in the database and on the Dashboard

[bensheldon]

Connects to #421.

• Adds a GoodJob::Projects ActiveRecord model
• Each process has a unique UUID that is updated on fork (when the PID changes)
• The record is managed within the Notifier using the same long-lived postgres connection that LISTEN/NOTIFY uses.
• An advisory lock is taken on the Process record (to take the place of a heartbeat/ttl) to detect dangling SIGKILLed processes.