fix broken clean_transmission plugin for seeding only torrent see #2803

by looking at the date_added time of the torrent. 
Date added should usually be less than date completed, but if one adds a torrent only for seeding (so all files are completed before adding the torrent) the date_finisehd field is not written.

This PR could be altered by someone more experienced to also remove seeding torrents after given timeframe.
Currently seeding only torrents are not deleted.