Planned:

- Add helper routines `server_time_from_client_time` and
  `client_time_from_server_time`
- Update documentation for ticket 134
- Possibly introduce keyword-only arguments (ticket 130)
- Improve caching (ticket 129)
- 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.
