*: Dial with handler and return handler on error and closed (#2191)

Require `NetworkBehaviourAction::{DialPeer,DialAddress}` to contain a
`ProtocolsHandler`. This allows a behaviour to attach custom state to its
handler. The behaviour would no longer need to track this state separately
during connection establishment, thus reducing state required in a behaviour.
E.g. in the case of `libp2p-kad` the behaviour can include a `GetRecord` request
in its handler, or e.g. in the case of `libp2p-request-response` the behaviour
can include the first request in the handler.

Return `ProtocolsHandler` on connection error and close. This allows a behaviour
to extract its custom state previously included in the handler on connection
failure and connection closing. E.g. in the case of `libp2p-kad` the behaviour
could extract the attached `GetRecord` from the handler of the failed connection
and then start another connection attempt with a new handler with the same
`GetRecord` or bubble up an error to the user.

Co-authored-by: Thomas Eizinger <thomas@eizinger.io>

core/src/connection.rs

@@ -229,10 +229,10 @@ where
         self.handler.inject_event(event);
     }
 
-    /// Begins an orderly shutdown of the connection, returning a
-    /// `Future` that resolves when connection shutdown is complete.
-    pub fn close(self) -> Close<TMuxer> {
-        self.muxing.close().0
+    /// Begins an orderly shutdown of the connection, returning the connection
+    /// handler and a `Future` that resolves when connection shutdown is complete.
+    pub fn close(self) -> (THandler, Close<TMuxer>) {
+        (self.handler, self.muxing.close().0)
     }
 
     /// Polls the connection for events produced by the associated handler