improved server usage in aspnet so no exceptions are thrown on startup

MQTTnet.Core/Server/IMqttServer.cs

@@ -8,6 +8,7 @@ namespace MQTTnet.Core.Server
     {
         event EventHandler<MqttClientConnectedEventArgs> ClientConnected;
         event EventHandler<MqttClientDisconnectedEventArgs> ClientDisconnected;
+        event EventHandler<MqttServerStartedEventArgs> Started;
 
         IList<ConnectedMqttClient> GetConnectedClients();
         void Publish(IEnumerable<MqttApplicationMessage> applicationMessages);

MQTTnet.Core/Server/MqttServer.cs

@@ -49,6 +49,7 @@ namespace MQTTnet.Core.Server
             return _clientSessionsManager.GetConnectedClients();
         }
 
+        public event EventHandler<MqttServerStartedEventArgs> Started;
         public event EventHandler<MqttClientConnectedEventArgs> ClientConnected;
         public event EventHandler<MqttClientDisconnectedEventArgs> ClientDisconnected;
         public event EventHandler<MqttApplicationMessageReceivedEventArgs> ApplicationMessageReceived;
@@ -96,6 +97,8 @@ namespace MQTTnet.Core.Server
             }
 
             _logger.LogInformation("Started.");
+
+            Started?.Invoke(this, new MqttServerStartedEventArgs());
         }
 
         public async Task StopAsync()

MQTTnet.Core/Server/MqttServerStartedEventArgs.cs

@@ -0,0 +1,8 @@
+using System;
+
+namespace MQTTnet.Core.Server
+{
+    public class MqttServerStartedEventArgs : EventArgs
+    {
+    }
+}