Improve logger performance.

Tests/MQTTnet.Benchmarks/ChannelAdapterBenchmark.cs

@@ -43,7 +43,7 @@ namespace MQTTnet.Benchmarks
 
             var channel = new TestMqttChannel(_stream);
             
-            _channelAdapter = new MqttChannelAdapter(channel, serializer, new MqttNetLogger().CreateChildLogger(nameof(MqttChannelAdapter)));
+            _channelAdapter = new MqttChannelAdapter(channel, serializer, new MqttNetLogger());
         }
 
         [Benchmark]

Tests/MQTTnet.Benchmarks/LoggerBenchmark.cs

@@ -9,15 +9,15 @@ namespace MQTTnet.Benchmarks
     [MemoryDiagnoser]
     public class LoggerBenchmark
     {
-        private IMqttNetLogger _logger;
-        private IMqttNetLogger _childLogger;
-        private bool _useHandler;
+        IMqttNetLogger _logger;
+        IMqttNetScopedLogger _childLogger;
+        bool _useHandler;
 
         [GlobalSetup]
         public void Setup()
         {
             _logger = new MqttNetLogger();
-            _childLogger = _logger.CreateChildLogger("child");
+            _childLogger = _logger.CreateScopedLogger("child");
 
             MqttNetGlobalLogger.LogMessagePublished += OnLogMessagePublished;
         }

Tests/MQTTnet.Benchmarks/SerializerBenchmark.cs

@@ -29,7 +29,7 @@ namespace MQTTnet.Benchmarks
                 Topic = "A"
             };
 
-            _serializer = new MqttV311PacketFormatter();
+            _serializer = new MqttV311PacketFormatter(new MqttPacketWriter());
             _serializedPacket = _serializer.Encode(_packet);
         }