Bug 1077136 - new async task to catch missing resultsets

ready for merge

@maurodoglio - would you do the honors?  :)
I think using cleanup_tasks.py for tasks is not standard. I like the idea of splitting the huge etl/tasks.py file into submodules though. A way to do that would be to create a tasks folder under etl and create as many modules as we want in there. Importing all those files inside etl/tasks/__init__.py will make celery pick them up as if they where all in the same file