Remove implementation of peer discovery

[not4x217]

Peer discovery must be implemented by application, built on top of CXO, hence:

• Discovery servers, based on gnet must be removed.
• Unfinished implementation of PEX must be removed.

Application, which uses CXO, has to implement application-specific wrapper on top of Node and Conn. That wrapper must include implementation of peer discovery (via PEX, DHT, tracking servers or any other mechanism).