dont use shutdownRef count for sync completion queue

src/csharp/Grpc.Core/Internal/CompletionQueueSafeHandle.cs

@@ -29,7 +29,7 @@ namespace Grpc.Core.Internal
     {
         static readonly NativeMethods Native = NativeMethods.Get();
 
-        AtomicCounter shutdownRefcount = new AtomicCounter(1);
+        AtomicCounter shutdownRefcount;
         CompletionRegistry completionRegistry;
 
         private CompletionQueueSafeHandle()
@@ -51,6 +51,7 @@ namespace Grpc.Core.Internal
         {
             var cq = Native.grpcsharp_completion_queue_create_async();
             cq.completionRegistry = completionRegistry;
+            cq.shutdownRefcount = new AtomicCounter(1);
             return cq;
         }
 
@@ -95,23 +96,29 @@ namespace Grpc.Core.Internal
 
         private void DecrementShutdownRefcount()
         {
-            if (shutdownRefcount.Decrement() == 0)
+            if (shutdownRefcount == null || shutdownRefcount.Decrement() == 0)
             {
                 Native.grpcsharp_completion_queue_shutdown(this);
             }
         }
 
         private void BeginOp()
+        {
+            if (shutdownRefcount != null)
             {
                 bool success = false;
                 shutdownRefcount.IncrementIfNonzero(ref success);
                 GrpcPreconditions.CheckState(success, "Shutdown has already been called");
             }
+        }
 
         private void EndOp()
+        {
+            if (shutdownRefcount != null)
             {
                 DecrementShutdownRefcount();
             }
+        }
 
         // Allows declaring BeginOp and EndOp of a completion queue with a using statement.
         // Declared as struct for better performance.