Fix fairness rounds indexing and enable last model fairness evaluation.

- Fairness rounds occur prior to training ones. Consequently, this
  commit modifies their indexing to be aligned with that of the
  _previous_ training round. As a result, utility and fairness
  metrics are properly aligned in checkpoint files.
- In addition, this commit makes it so that a final fairness
  evaluation round is run prior to ending the FL process when
  checkpointing is setup at the server level.