The synchonous add torrent method was replaced with async but this
break backward compatibility with 3rd party plugins and clients.
Added a new add_torrent_file_async method for adding single torrent.
Torrent manager has a new add_async method and split up code to prevent
duplication.
Update any use of add_torrent_file to add_torrent_file_async. Future
refactoring could use add_torrent_files instead.