Simplify concurrency with a proper actor framework

[jstuczyn]

Currently we have semi-unintentionally implemented our own 'actor' system. It'd be better to get rid of all that complexity and use something like actix or Riker.

However, that requires some additional research whether we should even do it - I will admit I only had a very quick glance at the concept itself.

[tommyv1987]

Closing issue, archiving for future planning session - the issues / tasks have moved to a new project board, so we can evaluate shortly.