Refactor session and connection handling in server. Fix QoS level 2 issues.

Tests/MQTTnet.Core.Tests/Mockups/TestServerExtensions.cs

@@ -0,0 +1,38 @@
+using System.Threading.Tasks;
+using MQTTnet.Client;
+using MQTTnet.Server;
+
+namespace MQTTnet.Tests.Mockups
+{
+    public static class TestServerExtensions
+    {
+        /// <summary>
+        /// publishes a message with a client and waits in the server until a message with the same topic is received
+        /// </summary>
+        /// <returns></returns>
+        public static async Task PublishAndWaitForAsync(this IMqttClient client, IMqttServer server, MqttApplicationMessage message)
+        {
+            var tcs = new TaskCompletionSource<object>();
+
+            void Handler(object sender, MqttApplicationMessageReceivedEventArgs args)
+            {
+                if (args.ApplicationMessage.Topic == message.Topic)
+                {
+                    tcs.SetResult(true);
+                }
+            }
+
+            server.ApplicationMessageReceived += Handler;
+
+            try
+            {
+                await client.PublishAsync(message).ConfigureAwait(false);
+                await tcs.Task.ConfigureAwait(false);
+            }
+            finally
+            {
+                server.ApplicationMessageReceived -= Handler;
+            }
+        }
+    }
+}