Avoid some full memory barriers (#1590)

* Avoid some full memory barriers

* Reset in stats itself

* Updated MqttClientStatistics

* Fixed order of members as artifact from merge-conflict

Source/MQTTnet/Internal/AsyncQueue.cs

@@ -22,7 +22,11 @@ namespace MQTTnet.Internal
 
         public void Clear()
         {
+#if NETCOREAPP3_1_OR_GREATER
+            _queue.Clear();
+#else
             Interlocked.Exchange(ref _queue, new ConcurrentQueue<TItem>());
+#endif
         }
 
         public void Dispose()
@@ -32,6 +36,16 @@ namespace MQTTnet.Internal
                 _signal.Dispose();
 
                 _isDisposed = true;
+
+#if !NETSTANDARD1_3
+                if (typeof(IDisposable).IsAssignableFrom(typeof(TItem)))
+                {
+                    while (_queue.TryDequeue(out TItem item))
+                    {
+                        (item as IDisposable).Dispose();
+                    }
+                }
+#endif
             }
         }