Use dispatch_after instead of usleep()

usleep() causes lag on the main thread.

How to test this:

1. Open the HelloMQTT example and run it on the simulator.

2. Press the Connect button. 

3. Turn off the WiFi. The HelloMQTT example forces a connection each time it receives the connection error status.   So in the console you should see the following error message. "connection error reconnecting..."

4. Interact with the UI and notice the lag.