Fix cleanup of client sessions

MQTTnet.Core/Adapter/MqttChannelCommunicationAdapter.cs

@@ -14,10 +14,10 @@ namespace MQTTnet.Core.Adapter
 {
     public class MqttChannelCommunicationAdapter : IMqttCommunicationAdapter
     {
+        private readonly SemaphoreSlim _semaphore = new SemaphoreSlim(1, 1);
         private readonly ILogger<MqttChannelCommunicationAdapter> _logger;
         private readonly IMqttCommunicationChannel _channel;
-        private readonly SemaphoreSlim _semaphore = new SemaphoreSlim(1, 1);
-
+        
         public MqttChannelCommunicationAdapter(IMqttCommunicationChannel channel, IMqttPacketSerializer serializer, ILogger<MqttChannelCommunicationAdapter> logger)
         {
             _logger = logger ?? throw new ArgumentNullException(nameof(logger));

MQTTnet.Core/Server/MqttClientRetainedMessagesManager.cs

@@ -68,7 +68,6 @@ namespace MQTTnet.Core.Server
 
             try
             {
-                // ReSharper disable once UseNullPropagation
                 if (_options.Storage != null)
                 {
                     await _options.Storage.SaveRetainedMessagesAsync(allRetainedMessages);

MQTTnet.Core/Server/MqttClientSession.cs

@@ -13,11 +13,11 @@ using Microsoft.Extensions.Options;
 
 namespace MQTTnet.Core.Server
 {
-    public sealed class MqttClientSession : IDisposable
+    public sealed class MqttClientSession
     {
-        private readonly IMqttClientRetainedMessageManager _clientRetainedMessageManager;
         private readonly HashSet<ushort> _unacknowledgedPublishPackets = new HashSet<ushort>();
 
+        private readonly IMqttClientRetainedMessageManager _clientRetainedMessageManager;
         private readonly MqttClientSubscriptionsManager _subscriptionsManager;
         private readonly MqttClientSessionsManager _sessionsManager;
         private readonly MqttClientPendingMessagesQueue _pendingMessagesQueue;
@@ -113,12 +113,6 @@ namespace MQTTnet.Core.Server
             _pendingMessagesQueue.Enqueue(publishPacket);
         }
 
-        public void Dispose()
-        {
-            _cancellationTokenSource?.Cancel();
-            _cancellationTokenSource?.Dispose();
-        }
-
         private async Task ReceivePacketsAsync(IMqttCommunicationAdapter adapter, CancellationToken cancellationToken)
         {
             try

MQTTnet.Core/Server/MqttClientSessionsManager.cs

@@ -164,10 +164,10 @@ namespace MQTTnet.Core.Server
                     if (connectPacket.CleanSession)
                     {
                         _clientSessions.Remove(connectPacket.ClientId);
-                        clientSession.Dispose();
+                        clientSession.Stop();
                         clientSession = null;
 
-                        _logger.LogTrace("Disposed existing session of client '{0}'.", connectPacket.ClientId);
+                        _logger.LogTrace("Stopped existing session of client '{0}'.", connectPacket.ClientId);
                     }
                     else
                     {