Issue background jobs stats

Implementation of the background job for refreshing the cached functions from site stats in a regular basis.
This is a first approach and tries to be as simple as possible in order not to add complexity before it is needed (that will happen, of course).
Good work. Quick thing: would be possible to copy and paste the code of warm.py directly here? I mean, warm.py does many more things than this functions, so I'm wondering what's missing. And I know @alejandrodob that you've a good answer about it :-)