refactor(swarm): don't use task_event_buffer_size for pending conne…

…ctions (#3187)

The task for a pending connection only ever sends one event into this channel: Either a success or a failure. Cloning a sender adds one slot to the capacity of the channel. Hence, we can start this capacity at 0 and have the `cloning` of the `Sender` take care of properly increasing the capacity.

swarm/src/connection/pool.rs

@@ -319,8 +319,7 @@ where
 {
     /// Creates a new empty `Pool`.
     pub fn new(local_id: PeerId, config: PoolConfig, limits: ConnectionLimits) -> Self {
-        let (pending_connection_events_tx, pending_connection_events_rx) =
-            mpsc::channel(config.task_event_buffer_size);
+        let (pending_connection_events_tx, pending_connection_events_rx) = mpsc::channel(0);
         let (established_connection_events_tx, established_connection_events_rx) =
             mpsc::channel(config.task_event_buffer_size);
         let executor = match config.executor {