p2p: improve peer selection logic
p2p: improve peer selection logic

This commit introduces a new (temporary) peer selection
strategy based on random lookups.

While we're here, also implement the TODOs in dialLoop.

maran/go-ethereum,maran/go-ethereum,maran/go-ethereum,maran/go-ethereum,maran/go-ethereum,maran/go-ethereum,maran/go-ethereum,maran/go-ethereum,maran/go-ethereum