Announce a new torrent on this tracker.
The fact that torrents must be announced here first makes this tracker a closed BitTorrent tracker: it will only accept clients for torrents it knows about, and this list of torrents is managed by the program instrumenting this Tracker class.
@param torrent The Torrent object to start tracking.
@return The torrent object for this torrent on this tracker. This may bedifferent from the supplied Torrent object if the tracker already contained a torrent with the same hash.