Fix iter_archive getting reset
The `DownloadManager.iter_archive` method currently returns an iterator - which is  **empty** once you iter over it once. This means you can't pass the same archive iterator to several splits.

To fix that, I changed the ouput of `DownloadManager.iter_archive` to be an iterable that you can iterate over several times, instead of a one-time-use iterator.

The `StreamingDownloadManager.iter_archive` already returns an appropriate iterable, and the code added in this PR is inspired from the one in `streaming_download_manager.py`