Cademlia and p2p peer pool (please dont merge)

[zelig]

initial untested commit for review @fjl
further architecture design coming in a comment

[fjl]

We had a long conversation and arrived at this. This is probably not going to be implemented, let's remember it for the future.

The idea is that we maintain a protocol-specific rating of peers in peer pools. We already
have the whisper.Whisper and eth.Blockpool objects which can serve as peer pools
(they know which peers have been good in the past). Swarm/TCP requires kademlia peers
for lookups and would also implement a pool. Peer pools would need to take care of
persistence internally, package p2p could provide common helpers for them.

The p2p.Server needs to find good
peers to connect to and can ask the protocol specific pools for good peers.
When new inbound peers connect or peer suggestions are received, package p2p would try
to add them to all pools. If none of them wants to keep the peer, it would be discarded.

What needs to be solved (which is also a general open question in the ethereum project) is
how to combine the requirements of all protocols to arrive at a good overall peer set.
A stupid initial implementation could just divide the available number of peer connections
into N sections (where N is the number of available pools) and fill each section from its
respective pool.

type Peer interface {
    Addr() net.Addr
    Pubkey() []byte
    LastActive() time.Time
}

type PeerPool interface {
    // AddPeer adds knowledge about a peer to the pool.
    // The pool should check whether this peer is
    // good and return an error if it considers
    // the peer unsuitable.
    AddPeer(Peer) error

    // GoodPeers fills the peers slice with suggestions
    // of peers that should be connected to, in order
    // of their goodness.
    // It returns the number of peers written.
    GoodPeers(peers []Peer) (n int)
}

type Protocol struct {
    Name string
    // ...and the other fields

    // PeerPool is the peer selection algorithm for
    // the protocol. It may be nil if the protocol
    // does not prefer a particular algorithm.
    PeerPool
}

[zelig]

Cademlia on wiki https://github.com/ethereum/wiki/wiki/Cademlia-Peer-Selection
For completeness sake, see what C++ guys have to say about this: https://github.com/ethereum/cpp-ethereum/wiki/RLPx-changes