Support IPV6 trackers.

Implemented BEP 7 "IPv6 Tracker Extension"

Tested with http://ipv6.torrent.ubuntu.com/

To test IPv6 using the ubuntu torrents I had to manually modify
startTrackerClient() to skip the first level of the announce list. The
Ubuntu torrents use an IPv4 tracker as the first level tracker, and
only use ipv6.torrent.ubuntu.com as the second level tracker.