Use a Kademlia instance per `ProtocolId`.

This PR changes discovery to use one kademlia instance per protocol
ID. The change is motivated by the fact that the single DHT we use
now includes incompatible nodes with different protocols.

To retain backwards compatibility we currently use a fake `ProtocolId`
matching the existing `/ipfs/kad/1.0.0` protocol string. If and when
this change is rolled out we can remove it and work solely with
`ProtocolId`s taken from the chain specification.