Add queue package

This is based on the [Ruby QueueClassic library](https://github.com/QueueClassic/queue_classic).

Opening now for early feedback.

Prerequisite for #536 
We cannot use PIDs to check inactivity due to the way database/sql maintains a pool of connections.

We should instead register each queue worker in discoverd with a uuid, and then unlock jobs locked by workers which don't appear in discoverd.