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

Add async handler interfaces for connected and disconnected to allow async code on connect

.. this allows a client to properly async publish on connect.

fix logger logId not used in ManagedMqttClient