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/modules/routing/avrorouter/avro_main.cc

@@ -335,7 +335,7 @@ static bool conversion_task_ctl(Avro *inst, bool start)
 
         if (task.get())
         {
-            worker->post(task);
+            worker->post(task, Worker::EXECUTE_AUTO);
             rval = true;
         }
     }