Separate job dispatching and queue working

[joseferben]

In order to support concurrent workers we need to split the job dispatching from the queue working. The challenge is to provide an ergonomic API that doesn't overwhelm beginners by forcing them to setup workers. The current implementation of the queue service contains the queue and the worker.

Proper locking with Postgres: https://www.2ndquadrant.com/en/blog/what-is-select-skip-locked-for-in-postgresql-9-5/