Refactor some fairness controllers code.

- Expose some subroutines under setup and fairness round,
  for the mere sake of making tests easier to perform, as
  well as to enable variants over current algorithms in
  the future / in experiments.
- Rename some methods and re-order some arguments.
- Refactor server-side aggregation of metrics, making it
  part of 'FairnessControllerServer' rather than part of
  'FederatedServer' backend code.