kad: Implement `FindManyNodesContext`

[lexnv]

The FindManyNodesContext was introduced in #77.
It currently generates a PUT_VALUE for the static list of provided peers.

Extend this functionality to drive multiple FindNodeContext queries.
When the user provides a peer who's address is not stored in the routing table, a FindNodeContext query should be executed.

This functionality is needed to keep parity with libp2p implementation.