Ability to seed 1 TByte of content using Tribler.

This requires announcing, say, 1000 swarms of content in the DHT.
This is a problem, as shown here: http://blog.libtorrent.org/2012/01/seeding-a-million-torrents/
The announce interval needs to be prolonged in order to reduce DHT announce traffic.
Perhaps the DHT cannot handle this and a new peer discovery method is required: #13.

As a quick partial fix we can put a cap on the maximum swarms to DHT announce. Then use a simple round-robin method to cycle slowly through all available swarms.