Determine whether it works to have multiple JobRunners on the same database

[FlixCoder]

It should mostly work, but currently it fetches all available jobs (#4). So, it wouldn't make a lot of sense to have multiple runners, because they don't split work effectively.

Would be interesting to know, if it works in general though, especially after concurrency limit is implemented.

[FlixCoder]

Concurrency limits is implemented, so more interesting now.