Fix Managed Client should dispose inner mqtt client #527

* Clean up

Added new models for review

Don't cancel token before stop session manager

First draft for MQTTnetServer project
* Basic support for settings
* Nasty code for setting up kestrel from appsettings.json

When no handlers are configured the client should not blow up

Add extension methods that provide a simpler api for IMqttClient and IManagedMqttClient