* Added interface IMqttRetainedMessagesManager to allow for different
retained messages manager implementations. The interface copies almost
exactly the current MqttRetainedMessagesManager implementation.
* Added IMqttRetainedMessagesManager.Start() to configure the provided
IMqttRetainedMessagesManager outside the constructor. This method returns
Task because some implementations that use external storage may be slow on
initialization (e.g., connect to a database).
* Modified MqttRetainedMessagesManager to implement new interface.
Federico Di Gregorio
Christian Kratky