MXS-2002 Make Worker excecution mode explicit

This is the first step in some cleanup of the Worker interface.
The execution mode must now be explicitly specified, but that is
just a temporary step. Further down the road, _posting_ will
*always* mean via the message loop while _executing_ will optionally
and by default mean direct execution if the calling thread is that
of the worker.

server/core/session.cc

@@ -359,7 +359,8 @@ static void session_free(MXS_SESSION *session)
     {
         // Destroy the service in the main routing worker thread
         mxs::RoutingWorker* main_worker = mxs::RoutingWorker::get(mxs::RoutingWorker::MAIN);
-        main_worker->post(std::auto_ptr<ServiceDestroyTask>(new ServiceDestroyTask(service)));
+        main_worker->post(std::auto_ptr<ServiceDestroyTask>(new ServiceDestroyTask(service)),
+                          mxs::Worker::EXECUTE_AUTO);
     }
 }