use raw buffer for frist connect msg then use buffered stream to read more efficient

Tests/MQTTnet.Core.Tests/MqttPacketSerializerTests.cs

@@ -393,6 +393,8 @@ namespace MQTTnet.Core.Tests
 
             public Stream ReceiveStream => _stream;
 
+            public Stream RawStream => _stream;
+
             public Stream SendStream => _stream;
 
             public bool IsConnected { get; } = true;

Tests/MQTTnet.Core.Tests/TestMqttCommunicationAdapter.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Concurrent;
 using System.Collections.Generic;
+using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Core.Adapter;
 using MQTTnet.Core.Client;
@@ -46,6 +47,11 @@ namespace MQTTnet.Core.Tests
             return Task.Run(() => _incomingPackets.Take());
         }
 
+        public IEnumerable<MqttBasePacket> ReceivePackets( CancellationToken cancellationToken )
+        {
+            return _incomingPackets.GetConsumingEnumerable();
+        }
+
         private void SendPacketInternal(MqttBasePacket packet)
         {
             if (packet == null) throw new ArgumentNullException(nameof(packet));