zikade: add private client-side handlers

[dettanym]

All references below are to the old private-go-libp2p-kad-dht repo, so find the appropriate ones in the private-zikade repo

1. v2/routing.go: Implements FindPeer and FindProvidersAsync methods. (Sidenote: interfaces includes these two methods are defined in the Go LibP2P core repo.) Both methods run d.kad.Query with the key as an argument and a function fn to check whether the node in the last hop has the correct CID.
2. DHT co-ordinator Query method: It looks up its own RT for the closest nodes and then currently, sends in a fake PB message with the key. So we need a private implementation of the Query method which does not send the key in the PB message (PIR request).