Download url to given path.

    Returns Promise -> sha256 of downloaded file.

    Args:
      url: address of resource to download.
      destination_path: `str`, path to directory where to download the resource.

    Returns:
      Promise obj -> (`str`, int): (downloaded object checksum, size in bytes).

def download(self, url, destination_path):
    """Download url to given path.

    Returns Promise -> sha256 of downloaded file.

    Args:
      url: address of resource to download.
      destination_path: `str`, path to directory where to download the resource.

    Returns:
      Promise obj -> (`str`, int): (downloaded object checksum, size in bytes).
    """
    self._pbar_url.update_total(1)
    future = self._executor.submit(self._sync_download, url, destination_path)
    return promise.Promise.resolve(future)