Add support for retained application messages

Frameworks/MQTTnet.NetStandard/Implementations/MqttServerAdapter.cs

@@ -24,7 +24,7 @@ namespace MQTTnet.Implementations
 
         public event EventHandler<MqttClientConnectedEventArgs> ClientConnected;
 
-        public void Start(MqttServerOptions options)
+        public Task StartAsync(MqttServerOptions options)
         {
             if (_isRunning) throw new InvalidOperationException("Server is already started.");
 
@@ -56,9 +56,11 @@ namespace MQTTnet.Implementations
 
                 Task.Run(() => AcceptTlsEndpointConnectionsAsync(_cancellationTokenSource.Token), _cancellationTokenSource.Token);
             }
+
+            return Task.FromResult(0);
         }
 
-        public void Stop()
+        public Task StopAsync()
         {
             _isRunning = false;
 
@@ -71,11 +73,13 @@ namespace MQTTnet.Implementations
 
             _tlsEndpointSocket?.Dispose();
             _tlsEndpointSocket = null;
+
+            return Task.FromResult(0);
         }
 
         public void Dispose()
         {
-            Stop();
+            StopAsync();
         }
 
         private async Task AcceptDefaultEndpointConnectionsAsync(CancellationToken cancellationToken)