Refactor namings of serializers etc.

Source/MQTTnet/Packets/MqttDisconnectPacket.cs

@@ -1,7 +1,15 @@
-namespace MQTTnet.Packets
+using System.Collections.Generic;
+using MQTTnet.Packets.Properties;
+
+namespace MQTTnet.Packets
 {
     public class MqttDisconnectPacket : MqttBasePacket
     {
+        /// <summary>
+        /// Added in MQTTv5.0.0.
+        /// </summary>
+        public List<IProperty> Properties { get; set; }
+
         public override string ToString()
         {
             return "Disconnect";