Add server events for subscribed and unsubscribed topics. Add pending messages count for connected clients.

Frameworks/MQTTnet.NetStandard/Server/MqttRetainedMessagesManager.cs

@@ -8,7 +8,7 @@ using MQTTnet.Packets;
 
 namespace MQTTnet.Server
 {
-    public sealed class MqttRetainedMessagesManager
+    public sealed class MqttRetainedMessagesManager : IDisposable
     {
         private readonly Dictionary<string, MqttApplicationMessage> _retainedMessages = new Dictionary<string, MqttApplicationMessage>();
         private readonly SemaphoreSlim _semaphore = new SemaphoreSlim(1, 1);
@@ -102,6 +102,11 @@ namespace MQTTnet.Server
             return retainedMessages;
         }
 
+        public void Dispose()
+        {
+            _semaphore?.Dispose();
+        }
+
         private async Task HandleMessageInternalAsync(string clientId, MqttApplicationMessage applicationMessage)
         {
             var saveIsRequired = false;