Add DHT key-value behaviour (#2937)

* Add DHT key-value behaviour

* Apply suggestions from code review

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

* Apply suggestions from code review

Co-Authored-By: Pierre Krieger <pierre.krieger1708@gmail.com>

* Return which key failed to be inserted

substrate/core/network/src/test/mod.rs

@@ -196,6 +196,11 @@ impl NetworkSpecialization<Block> for DummySpecialization {
 		_peer_id: PeerId,
 		_message: &mut Option<crate::message::Message<Block>>,
 	) {}
+
+	fn on_event(
+		&mut self,
+		_event: crate::event::Event
+	) {}
 }
 
 pub type PeersFullClient =