Multi-db integration

[64kramsystem]

Although the project will be focused on MySQL at the beginning, we should design from the beginning for difference DB backends.

The task of this issue is to investigate how we can start from the beginning, implementing multiple backends.

I have no idea how to do this, and which reference projects are out there, so it's a very open task.

I don't even know if the instance_events itself can be written in a db-independent way, so that should be the topic.

Once that's investigated, we'll start writing the same base, and also proceed with the first db-dependent logic (queue processing).