title: Celery: How to get the task completed time from AsyncResult
body: <p>I need to trace the status for the tasks. i could get the 'state', 'info' attribute from the AsyncResult obj. however, it looks there's no way to get the 'done_date'. I use MySQL as result backend so i could find the <code>date_done</code> column in the <code>taskmeta</code> table, but how could i get the task done date directly from AysncResult obj? thanks</p>