Parallel programmatic adding of torrents causes torrents to be skipped

[krichter]

Adding approx. 50 torrents programmatically in a multithreaded Python script causes not all of them being added. The script at ​https://github.com/krichter722/freebsd-magnet-links-parser makes a good example/case for reproducation (save the text of ​https://wiki.freebsd.org/Torrents into a file and comment out the threading code).

experienced with 2.0.0.dev7097+e3f5377+201707051516~ubuntu17.04.1 on Ubuntu 17.04