Rewrite the libp2p networking (#742)

* Rewrite the libp2p networking

* Fix erroneous replacement during rebase

* Update libp2p

* Update libp2p

* Remove the logic error about useless substreams

* Use the new NodeHandler system of libp2p

* Commit the Cargo.lock

* Upgrade yamux

* Forward port latest changes

* Fix compilation issues

* Implement the external URL

* Update to latest networking fixes

* Forward port rest of v0.2

* Fix reserved peers being dropped when we're full