Adding support for .NET Standard

Added TLS1.2 support to client and server.

Instead of awaiting most tasks, changed to simply returning the tasks.

Added MqttClientSslChannel and MqttServerSslChannel which both utilize
an SSL stream.  Added options to both MqttCLientOptions and
MqttServerOptions.  I was unable to update the code in
MQTTnet.UniversalWindows due to my lack of Windows 10.

* [Server] Fixed an issue with not correctly removed old client sessions.
* [Server] Several minor performance improvements.
* [Server] An existing client session is no longer closed if a new client connection is invalid.
* [Client] Added support for sending "CleanSession" flag.