Refactor serializer locking and thread instances.

Source/MQTTnet/Adapter/MqttChannelAdapter.cs

@@ -18,6 +18,8 @@ namespace MQTTnet.Adapter
         private const uint ErrorOperationAborted = 0x800703E3;
         private const int ReadBufferSize = 4096;  // TODO: Move buffer size to config
 
+        private readonly SemaphoreSlim _writerSemaphore = new SemaphoreSlim(1, 1);
+
         private readonly IMqttNetChildLogger _logger;
         private readonly IMqttChannel _channel;
 
@@ -88,6 +90,7 @@ namespace MQTTnet.Adapter
 
         public async Task SendPacketAsync(MqttBasePacket packet, CancellationToken cancellationToken)
         {
+            await _writerSemaphore.WaitAsync(cancellationToken).ConfigureAwait(false);
             try
             {
                 _logger.Verbose("TX >>> {0}", packet);
@@ -107,6 +110,10 @@ namespace MQTTnet.Adapter
 
                 WrapException(exception);
             }
+            finally
+            {
+                _writerSemaphore.Release();
+            }
         }
 
         public async Task<MqttBasePacket> ReceivePacketAsync(TimeSpan timeout, CancellationToken cancellationToken)