MXS-2002 Rename Worker::post() to Worker::execute()

The main point is that tasks/functions are executed, not that
they are posted.

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(std::move(task), Worker::EXECUTE_AUTO);
+            worker->execute(std::move(task), Worker::EXECUTE_AUTO);
             rval = true;
         }
     }