* [Server] Added support for complex client IDs.

* [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.

Tests/MQTTnet.TestMqttClient/Program.cs

@@ -33,7 +33,9 @@ namespace MQTTnet.TestMqttClient
             {
                 var options = new MqttClientOptions
                 {
-                    Server = "localhost"
+                    Server = "localhost",
+                    ClientId = "XYZ",
+                    CleanSession = true
                 };
 
                 var client = new MqttClientFactory().CreateMqttClient(options);