Implement a FOSS & offline-first message broker to queue tasks

[rudokemper]

Feature Request

Right now, mapgl-tile-renderer is set up to work with Azure Storage Queue as a message broker to receive tasks to generate offline maps in the background, and send a message back to remove that task from the queue once completed.

Let's provide an open-source alternative so we are not exclusively tied to Azure. One possibility is to use RabbitMQ.

[rudokemper]

Alternatively, we can consider taking this approach of co-locating the task queue in the database directly, which would eliminate the need for a FOSS / offline-first message broker. Cf. #50 for more discussion.

[IamJeffG]

I have used the in-DB approach in other systems. It works great and I highly recommend it for the simplicity. Introducing a different system (e.g. RabbitMQ) is prob worth it only if you have enormous volume of messages going across.