Switch from devp2p to libp2p (#268)

* Switch from devp2p to libp2p

* Move the keys in the network state

* Properly load, store or generate private key

* Some robustness

* Update for latest libp2p

* Allow secio

* Don't open a new Kademlia connec all the time

* Handle Kademlia disconnection

* Set correct permissions on key file

* Improvements to secret key storage

* Flush the peer store at Kademlia requests

* Use RAII guards for disconnection

* Some misc work

* Set informations about peers

* Fix tests and external URL

* Fix some style

* Split obtain_private_key into multiple function

* More style fixes

* More style fixes

* Fix some concerns

* Turn // into ///

* More style fixes

* More style fixes

* Add annotations to unreachable!

* Fix style again

* Remove commented out code

* Fix test year

* More concerns