Update API

- Add `transfer_state` enum with appropriate values for a transfer
  current status.
- Add `transfer_status` class with functions `state()`, `done()`,
  `failed()` and `error()` to retrieve a transfer's current status and
  its associated error condition in case of failure.
- Extend the `transfer` class with functions `wait()`, and `wait_for()`,
  to wait for the completion of a transfer request.
- Add new error codes: `no_such_transfer` and `transfer_in_progress`.
- Add new formatters: `transfer_state` and `std::optional<T>`.
- Add new API function: `transfer_dataset` as a convenience function to
  request the transfer of a single dataset.