Fixed a race condition in client and server.

Tests/MQTTnet.Core.Tests/TestMqttCommunicationAdapter.cs

@@ -17,6 +17,10 @@ namespace MQTTnet.Core.Tests
 
         public IMqttPacketSerializer PacketSerializer { get; } = new MqttPacketSerializer();
 
+        public void Dispose()
+        {
+        }
+
         public Task ConnectAsync(TimeSpan timeout)
         {
             return Task.FromResult(0);

Tests/MQTTnet.TestApp.NetCore/ServerTest.cs

@@ -89,7 +89,7 @@ namespace MQTTnet.TestApp.NetCore
                             c.ApplicationMessage.Payload = Encoding.UTF8.GetBytes(content.ToString());
                         }
                     }
-                    catch (Exception e)
+                    catch (Exception)
                     {
                     }
                 };