Improve Fairness integration into the main orchestrating classes.

* Integrate Fairness configuration on the server side, split
  between 'FLOptimConfig' and 'FLRunConfig'.
* Implement generic fairness measures computation on the server
  side. Implement default computations on the client side. Leave
  additional actions up to subclasses.
  This will potentially be refactored to be coherent as to what
  goes in the fairness controllers and what goes in the existing
  main classes, and as to what is part of the shared API and what
  is left to algorithm-specific subclasses.
* Move fairness round query and reply messages to 'messaging'.