feat(swarm)!: allow NetworkBehaviours to manage connections

Previously, a `ConnectionHandler` was immediately requested from the `NetworkBehaviour` as soon as a new dial was initiated or a new incoming connection accepted.

With this patch, we delay the creation of the handler until the connection is actually established and fully upgraded, i.e authenticated and multiplexed.

As a consequence, `NetworkBehaviour::new_handler` is now deprecated in favor of a new set of callbacks:

- `NetworkBehaviour::handle_pending_inbound_connection`
- `NetworkBehaviour::handle_pending_outbound_connection`
- `NetworkBehaviour::handle_established_inbound_connection`
- `NetworkBehaviour::handle_established_outbound_connection`

All callbacks are fallible, allowing the `NetworkBehaviour` to abort the connection either immediately or after it is fully established. All callbacks also receive a `ConnectionId` parameter which uniquely identifies the connection. For example, in case a `NetworkBehaviour` issues a dial via `NetworkBehaviourAction::Dial`, it can unambiguously detect this dial in these lifecycle callbacks via the `ConnectionId`.

Finally, `NetworkBehaviour::handle_pending_outbound_connection` also replaces `NetworkBehaviour::addresses_of_peer` by allowing the behaviour to return more addresses to be used for the dial.

Resolves #2824.

Pull-Request: #3254.

swarm/src/keep_alive.rs

@@ -4,10 +4,9 @@ use crate::handler::{
     ConnectionEvent, ConnectionHandlerEvent, FullyNegotiatedInbound, FullyNegotiatedOutbound,
     KeepAlive, SubstreamProtocol,
 };
-use crate::THandlerInEvent;
-use crate::THandlerOutEvent;
+use crate::{ConnectionDenied, THandler, THandlerInEvent, THandlerOutEvent};
 use libp2p_core::upgrade::DeniedUpgrade;
-use libp2p_core::PeerId;
+use libp2p_core::{Endpoint, Multiaddr, PeerId};
 use std::task::{Context, Poll};
 use void::Void;
 
@@ -24,8 +23,24 @@ impl NetworkBehaviour for Behaviour {
     type ConnectionHandler = ConnectionHandler;
     type OutEvent = Void;
 
-    fn new_handler(&mut self) -> Self::ConnectionHandler {
-        ConnectionHandler
+    fn handle_established_inbound_connection(
+        &mut self,
+        _: ConnectionId,
+        _: PeerId,
+        _: &Multiaddr,
+        _: &Multiaddr,
+    ) -> Result<THandler<Self>, ConnectionDenied> {
+        Ok(ConnectionHandler)
+    }
+
+    fn handle_established_outbound_connection(
+        &mut self,
+        _: ConnectionId,
+        _: PeerId,
+        _: &Multiaddr,
+        _: Endpoint,
+    ) -> Result<THandler<Self>, ConnectionDenied> {
+        Ok(ConnectionHandler)
     }
 
     fn on_connection_handler_event(