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/protocol.rs

@@ -30,6 +30,7 @@ use message::{
 };
 use message::{BlockAttributes, Direction, FromBlock, RequestId};
 use message::generic::{Message as GenericMessage, ConsensusMessage};
+use event::Event;
 use consensus_gossip::{ConsensusGossip, MessageRecipient as GossipMessageRecipient};
 use on_demand::{OnDemandCore, OnDemandNetwork, RequestData};
 use specialization::NetworkSpecialization;
@@ -49,6 +50,7 @@ use util::LruHashSet;
 mod util;
 pub mod consensus_gossip;
 pub mod message;
+pub mod event;
 pub mod on_demand;
 pub mod specialization;
 pub mod sync;
@@ -497,6 +499,10 @@ impl<B: BlockT, S: NetworkSpecialization<B>, H: ExHashT> Protocol<B, S, H> {
 		self.context_data.peers.iter().map(|(id, peer)| (id, &peer.info))
 	}
 
+	pub fn on_event(&mut self, event: Event) {
+		self.specialization.on_event(event);
+	}
+
 	pub fn on_custom_message(
 		&mut self,
 		network_out: &mut dyn NetworkOut<B>,