Refactoring minor issues

Frameworks/MQTTnet.NetStandard/Server/MqttClientPendingMessagesQueue.cs

@@ -1,6 +1,5 @@
 using System;
 using System.Collections.Concurrent;
-using System.Runtime.CompilerServices;
 using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Adapter;

Frameworks/MQTTnet.NetStandard/Server/MqttClientSessionsManager.cs

@@ -187,7 +187,7 @@ namespace MQTTnet.Server
             {
                 foreach (var clientSession in _sessions.Values)
                 {
-                    await clientSession.EnqueueApplicationMessageAsync(applicationMessage);
+                    await clientSession.EnqueueApplicationMessageAsync(applicationMessage).ConfigureAwait(false);
                 }
             }
             finally

Frameworks/MQTTnet.NetStandard/Server/MqttClientSubscriptionsManager.cs

@@ -10,8 +10,8 @@ namespace MQTTnet.Server
 {
     public sealed class MqttClientSubscriptionsManager : IDisposable
     {
-        private readonly SemaphoreSlim _semaphore = new SemaphoreSlim(1, 1);
         private readonly Dictionary<string, MqttQualityOfServiceLevel> _subscriptions = new Dictionary<string, MqttQualityOfServiceLevel>();
+        private readonly SemaphoreSlim _semaphore = new SemaphoreSlim(1, 1);
         private readonly IMqttServerOptions _options;
         private readonly string _clientId;