Added custom exception for gracefully closed exceptions.

Source/MQTTnet/Exceptions/MqttCommunicationClosedGracefullyException.cs

@@ -0,0 +1,6 @@
+namespace MQTTnet.Exceptions
+{
+    public class MqttCommunicationClosedGracefullyException : MqttCommunicationException
+    {
+    }
+}

Source/MQTTnet/Implementations/MqttTcpServerListener.cs

@@ -76,6 +76,8 @@ namespace MQTTnet.Implementations
                         await sslStream.AuthenticateAsServerAsync(_tlsCertificate, false, SslProtocols.Tls12, false).ConfigureAwait(false);
                     }
 
+                    _logger.Verbose($"Client '{clientSocket.RemoteEndPoint}' accepted by TCP listener '{_socket.LocalEndPoint}, {_addressFamily}'.");
+
                     var clientAdapter = new MqttChannelAdapter(new MqttTcpChannel(clientSocket, null), new MqttPacketSerializer(), _logger);
                     ClientAccepted?.Invoke(this, new MqttServerAdapterClientAcceptedEventArgs(clientAdapter));
                 }

Source/MQTTnet/Internal/ExceptionHelper.cs

@@ -6,7 +6,7 @@ namespace MQTTnet.Internal
     {
         public static void ThrowGracefulSocketClose()
         {
-            throw new MqttCommunicationException("Connection gracefully closed from the remote party.");
+            throw new MqttCommunicationClosedGracefullyException();
         }
     }
 }

Source/MQTTnet/Server/MqttClientSession.cs

@@ -100,7 +100,14 @@ namespace MQTTnet.Server
             {
                 if (exception is MqttCommunicationException)
                 {
-                    _logger.Warning(exception, "Client '{0}': Communication exception while receiving client packets.", ClientId);
+                    if (exception is MqttCommunicationClosedGracefullyException)
+                    {
+                        _logger.Verbose("Client '{0}': Connection closed gracefully.", ClientId); ;
+                    }
+                    else
+                    {
+                        _logger.Warning(exception, "Client '{0}': Communication exception while receiving client packets.", ClientId);
+                    }
                 }
                 else
                 {