Planned:

- Improve sync code.


Ideas for future development:

- See ticket system

- Use `freezegun` module to speed up long-running test for
  `upload_if_newer` and `download_if_newer`. Note: It won't be
  possible, to speed up `test_public_servers.py` with this.

- Support `FTPFile.seek` and `FTPFile.tell` as far as it makes sense.

- WebDAV support?
