fix modelshutown corner case

fix commit cff0c3d did solve the semaphore disposed exception, however, there is a bug that prevents the  modelshutdown work from being exectuted and the registered handler from being called.

Author: Petros Argyrakis

projects/RabbitMQ.Client/client/impl/AsyncConsumerWorkService.cs

@@ -128,7 +128,7 @@ private async Task LoopWithConcurrency(CancellationToken cancellationToken)
                 {
                     while (await _channel.Reader.WaitToReadAsync(cancellationToken).ConfigureAwait(false))
                     {
-                        while (_channel.Reader.TryRead(out Work work) && !cancellationToken.IsCancellationRequested)
+                        while (_channel.Reader.TryRead(out Work work))
                         {
                             // Do a quick synchronous check before we resort to async/await with the state-machine overhead.
                             if (!_limiter.Wait(0))
@@ -144,6 +144,10 @@ private async Task LoopWithConcurrency(CancellationToken cancellationToken)
                 {
                     // ignored
                 }
+                finally
+                {
+                    _limiter?.Dispose();
+                }
             }
 
             private static async Task HandleConcurrent(Work work, IModel model, SemaphoreSlim limiter)
@@ -186,7 +190,6 @@ public Task Stop()
             {
                 _channel.Writer.Complete();
                 _tokenSource?.Cancel();
-                _limiter?.Dispose();
                 return _worker;
             }
         }