2.1.0-29

• update to libtorrent master
• fix fetch magnet, save the add_torrent_params as a torrent file
• lt:add need_save_resume field to torrent_status, with the resume data flags
  indicating which aspects of a torrent have changed since the last resume
  data was saved. Also make only_if_modified flag a combination of the other
  flags.
• lt:separate only_if_modified flag to save_resume_data
• lt:removing an ephemeral web seed does not affect whether resume data needs
  to be saved
• lt:allow promoting an ephemeral web seed when adding it
• lt:fix web seed request for renamed single-file torrents
• lt:instead of removing web seed with certain errors, just disable them and
  preserve them in the resume data.
• lt:improve behavior of save_resume_data() only_if_modified flag to avoid
  re-saving the resume data too frequently
• lt:port 1 is used as a placeholder for announcing to trackers without the
  ability to accept incoming connections
• lt:fix issue in retrying trackers in tiers > 0
• lt:fix time_point conversion from/to time_t
• lt:don't throw a std::system_error holding a boost::system::error_code
• lt:source code cleanup, performance and stability