BEP 0027 initial implementation https://www.bittorrent.org/beps/bep_0027.html

The intention is mainly to at least stop misbehaving when facing private torrents, i.e.:

• don't use DHT
• don't announce on multiple trackers at once

Done these:

• private field is serialized and deserialized properly
• DHT is disabled for private trackers
• only the first tracker is used from the list (to simplify implementation). The algorithm in BEP 27 is a little more involved when handling multiple trackers.