Support private torrents (BEP 27)

[av-gal]

Entails checking the torrent's infodict for a "private" boolean, and disabling DHT/PEX for that torrent if the flag is set to true. If a private torrent lists multiple trackers, only one tracker from the list should be used at a time. The other trackers should be used as backups, and all peers should be dropped when switching from one tracker to the other.

Supporting private torrents would make it possible to use rqbit with torrent trackers that restrict access to the swarm from the general public.