Usage:

```js
socket.timeout(5000).emit("my-event", (err) => {
  if (err) {
    // the server did not acknowledge the event in the given delay
  }
});
```

Related: socketio/socket.io#4150

Note: the version must be kept in sync when publishing a new release

Related: #1513

Previously, calling `socket.disconnect().connect()` could, if the
connection was upgraded to WebSocket, result in "disconnect" being
emitted twice, and an engine being leaked.

Here's what happened:

> socket.disconnect()

- calls `socket.destroy()` so the socket doesn't listen to the manager events anymore
- then calls `manager._close()` which closes the underlying engine but not the manager itself (it waits for the "close" event of the engine)

> socket.connect()

- calls `socket.subEvents()` so the socket does listen to the manager events
- calls `manager.open()` which creates a new engine

And then the first engine emits a "close" event, which is forwarded to
the socket, hence the second "disconnect" event.

Related: #1014

The ack was not properly removed upon timeout, and could be called
twice.

Related: ccf7998

Diff: socketio/engine.io-client@6.0.1...6.1.1

Diff: 4.3.2...4.4.0