validator-discovery: basic retrying logic (#3059)

* validator_discovery: less flexible, but simpler design

* fix test

* remove unused struct

* smol optimization

* validator_discovery: basic retrying logic

* add a test

* add more tests

* update the guide

* more test logic

* Require at least 2/3 connectivity.

* Fix test.

* Update node/network/gossip-support/src/lib.rs

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

* Update node/network/gossip-support/src/lib.rs

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

Co-authored-by: Robert Klotzner <robert.klotzner@gmx.at>
Co-authored-by: Robert Klotzner <eskimor@users.noreply.github.com>
Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

polkadot/node/subsystem/src/messages.rs

@@ -253,6 +253,9 @@ pub enum NetworkBridgeMessage {
 		validator_ids: Vec<AuthorityDiscoveryId>,
 		/// The underlying protocol to use for this request.
 		peer_set: PeerSet,
+		/// Sends back the number of `AuthorityDiscoveryId`s which
+		/// authority discovery has failed to resolve.
+		failed: oneshot::Sender<usize>,
 	},
 }