fix(eio): discard all pending packets when the server is closed

In some specific cases, the transport was not closed right away,
leaving the Node.js process alive even after closing the server.
The HTTP long-polling transport would be closed after the heartbeat
failure and the `closeTimeout` delay (20 + 25 + 30 seconds).

Example:

```js
io.on("connection", (socket) => {
  // the writeBuffer is not empty, so the transport is not closed right away
  io.close();
});
```

Related: https://github.com/socketio/socket.io/issues/5088