Dispersy: peer discovery community (no more initial bootstrap servers)

[synctext]

To bootstrap the search, allchannels and other communities we currently require connectable servers.

The aim of this enhancement is that all connectable peers automatically form a community bootstrap overlay. An alternative is to use our DHT, but connectability and lack of NAT-awareness make this less attractive. Similar to the Kazaa 2001 and Buddycast 2005 design we probably better use a local host-cache with the 200+ peers which we've seen in every community.

The Tribler installer will no longer include a hard-coded list of 16 bootstrap servers, but a "peer_cache.txt" with a few hundred peers to find the network. On a daily basis we refresh this peer_cache with stable connectable peers. Plus we put this updated file in the Linux/Mac/Windows installers.

[synctext]

As commented we also need this to do a "No-Internet-Needed" smartphone app

[NielsZeilemaker]

For the No-Internet-Needed stuff we need to be able to discover peers without having to rely on any tracker.
The peer discovery community still requires at least a tracker to find peers which are in the peer discovery community.

[synctext]

Some history, in March 2006 we already worked on this stuff: http://web.archive.org/web/20060429043719/http://tribler.org/developers/?layer0=105

[NielsZeilemaker]

Discovery community and peercache.txt files are implemented/integrated

[synctext]

See: Tribler/dispersy#81
Code: https://github.com/Tribler/dispersy/blob/devel/discovery/bootstrap.py

[whirm]

@synctext but we aren't bundling one with Tribler yet, so the bootstrap servers are still needed.