Only one thread for short periods of time

(migrated from: https://gitlab.com/christophfink/flickrhistory/-/issues/17)

When each gap in the already collected history is shorter than 1 day, the entire downloader runs on one thread only

Would be a quick and easy fix: in `basicflickrhistorydownloader.py:169ff`, we treat short gaps different than long gaps - which we should not, after all.