ManagedMqttClient

PublishAsync is now EnqueueAsync, but while PublishAsync used to accept a cancellation token, EnqueueAsync does not. I've had cases where PublishAsync used to get stuck, so I used to cancel it after 7 seconds. Should we use something like this?

````
var ct = new CancellationTokenSource(TimeSpan.FromSeconds(7)).Token;
await client.EnqueueAsync(message).WaitAsync(ct).ConfigureAwait(false);