- make MqttPacketReader and Writer a specialized version of their Binary counterparts

- improved message framing. as basis to improve buffering
- share more common code for the writer part

MQTTnet.Core/Packets/MqttPacketHeader.cs

@@ -0,0 +1,13 @@
+using MQTTnet.Core.Protocol;
+
+namespace MQTTnet.Core.Packets
+{
+    public class MqttPacketHeader
+    {
+        public MqttControlPacketType ControlPacketType { get; set; }
+
+        public byte FixedHeader { get; set; }
+
+        public int BodyLength { get; set; }
+    }
+}