Send a message instead of dropping the endpoint

Makes the intent clearer, but also fixes a bug because the receiver
endpoint doesn't handle close errors correctly.

zebra-network/src/peer_set/set.rs

@@ -492,7 +492,7 @@ where
         } else {
             // Cancel any request made to the service because it is using an outdated protocol
             // version.
-            std::mem::drop(tx);
+            let _ = tx.send(CancelClientWork);
         }
     }