Multi protocol peer selection

In order for the network emerging from the peer selection algorithm in p2p package to support multiple protocols that might require routing, such as Swarm, the peer set must satisfy the following properties:

• Peers should make it known which protocols they support (as a list of identifiers)
• In each row of the Kademlia table, there should be a sufficient number (as defined by the redundancy requirement k, which might be distinct for each protocol) of peers supporting each of the protocols which the node itself supports.

The second requirement is achieved by maintaining distinct redundancy limits kp for each protocol p, with peers supporting multiple protocols counting towards the corresponding limints for each of them.

Thus, even nodes that do not support a specific protocol can help other nodes in search of peers that do support it.