Rethink mquery's configuration about mquery

[msm-code]

Thanks for creating this issue! I've already wrote half a blog post in the linked issue, so just few initial thoughts:

don't get rid of redis but try to use it for caching and possibly plugin config

Just to be clear, redis is used for everything including job objets and lists of results. So it's way more than caching. I've added documentation for all data that is stored in reds a few days ago, but it's not merged yet: https://github.com/CERT-Polska/mquery/blob/5dc1070db34bbb4ee39c9e6157403564eb526c31/docs/redis.md

Currently what I have in mind is to move core config to a separate config file

I quite like https://github.com/bwindsor/typed-config (this is what's used by mwdb-core - https://github.com/CERT-Polska/mwdb-core/blob/master/mwdb/core/config.py). If we decide to rewrite config handling, maybe it's a good project to use.