libp2p-next (#3076)

* Changes for the next libp2p release:

  * Updates to the Kademlia APIs.
  * Updated imports due to the extracted libp2p-swarm crate.
  * ...

Still pending at least the following:

  * rust-libp2p/#1189
  * rust-libp2p/#1191
  * rust-libp2p/#1194

* Use Quorum::One.

The previous choice was apparently arbitrary.

* Use libp2p-0.11 from crates.io. Address feedback.

* Correct imports after merge.