Add a substrate-peerset crate (#2042)

* Add a substrate-peerset crate

* Some adjustements

* More adjustements

* Use a temporary libp2p branch

* Add back-off mechanism

* Fix RPC tests

* Some adjustements

* Another libp2p bugfix

* Do a round-robin in the peerset

* Use a real dependency instead of a patch for libp2p

* Initialize reserved nodes correctly

* Better diagnostic for no address

* Don't allocate slots if in reserved only

* Ban node on dial failure

* Fix indentation