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 known node 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 node supporting each of the protocols which the host itself supports.

The second requirement is achieved by maintaining distinct redundancy limits k_p for each protocol p, with nodes supporting multiple protocols counting towards the corresponding limits 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.