Destroy workers after services

The workers need to be destroyed only after services have been
to ensure that they are around in case the destruction of services
involves the closing of dcbs.

server/core/gateway.cc

@@ -2139,14 +2139,14 @@ int main(int argc, char **argv)
         worker->join();
     }
 
-    Worker::finish();
-    MessageQueue::finish();
-
     /*<
      * Destroy the router and filter instances of all services.
      */
     service_destroy_instances();
 
+    Worker::finish();
+    MessageQueue::finish();
+
     /*<
      * Wait the flush thread.
      */