Fix redundant model weights sharing with clients.

- Until now, in the main FL process, clients would always receive
  global model weights twice, due to their being shared both with
  training and evaluation requests.
- This commit fixes that via the introduction of backend mechanisms
  to keep track of which clients hold the latest model weights, and
  sparingly attach them to requests based on this.
- As a side effect, `TrainRequest` and `EvaluationRequest` messages
  now specify `weights` to be an `Optional[Vector]` field.