Refactor trace messages

MQTTnet.Core/Packets/MqttUnsubAckPacket.cs

@@ -3,5 +3,10 @@
     public sealed class MqttUnsubAckPacket : MqttBasePacket, IMqttPacketWithIdentifier
     {
         public ushort PacketIdentifier { get; set; }
+
+        public override string ToString()
+        {
+            return "UnsubAck: [PacketIdentifier=" + PacketIdentifier + "]";
+        }
     }
 }