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/resource.cc

@@ -1280,7 +1280,7 @@ HttpResponse resource_handle_request(const HttpRequest& request)
     mxs::Semaphore sem;
     ResourceTask task(request);
 
-    worker->post(&task, &sem);
+    worker->post(&task, &sem, mxs::Worker::EXECUTE_AUTO);
     sem.wait();
 
     return task.result();