Implement lpd

As should be defined in BEP-0014 (still TBD)

Basically, for each torrent we are in, a HTTP-like message is sent to
239.192.152.143:6771. This message is an announcement that we are in
the swarm.

When a Taipei-Torrent peer receives such a message, it will
automatically connect unless it's already connected.

Tested successfully with aria2c and transmission.