Stop() & IsRunning() methods on the uMQTTBroker

[amarendrapramanik]

Requesting to implement the following two methods Stop() & IsRunning() methods on the uMQTTBroker is sometimes very important specially when someone want to stop the server on certain conditions. Also IsRunning is important when implementing any watchdog to check periodically if the server is running, if not init() once more time.