Send sub-tasks to celery properly, even post_request_tasks (bug 1021158)

post_request_task before this patch only send tasks to celery inside
the request-response cycle, preventing indexation from being triggered
from inside a task (outside the request-response cycle).
Merge pull request #2211 from diox/tasks-inception

Send sub-tasks to celery properly, even post_request_tasks (bug 1021158)