fix: only set 'connected' to true after middleware execution

The Socket instance is only considered connected when the "connection"
event is emitted, and not during the middleware(s) execution.

```js
io.use((socket, next) => {
  console.log(socket.connected); // prints "false"
  next();
});

io.on("connection", (socket) => {
  console.log(socket.connected); // prints "true"
});
```

Related: https://github.com/socketio/socket.io/issues/4129

Backported from https://github.com/socketio/socket.io/commit/02b0f73e2c64b09c72c5fbf7dc5f059557bdbe50