* [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.

MQTTnet.Core/Server/GetOrCreateClientSessionResult.cs

@@ -0,0 +1,9 @@
+namespace MQTTnet.Core.Server
+{
+    public class GetOrCreateClientSessionResult
+    {
+        public bool IsExistingSession { get; set; }
+
+        public MqttClientSession Session { get; set; }
+    }
+}