Revise fairness controllers API and integration.

- Revise the way client-side controllers are instantiated from
  server-emitted instructions, using the usual type-registration
  tools and removing the need to subclass setup query messages.
- Move fairness-related base messages to the 'messaging' module
  (and 'secagg.messaging' one).
- Have the client-side controller access and wrap a training
  manager at instantiation. Revise method signatures and docs.
- Add fairness metrics checkpointing.