Also add comment in task_executor.h that implementations for scheduleWork/scheduleWorkAt/scheduleRemoteCommand should ensure that the passed callback is the last thing that gets called (no post processing directly related to the callback request).

orig title: AsyncRequestsSender::_ready should wait for callback to finish running before returning data
orig description: Currently, it just clears the handle from the executor and never explicitly waits for it to finish. It looks like AsyncResultsMerger may also have the same issue.