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).
Would be possible to use the app.logger.warning method?

```python
app.logger.warning("This job is already scheduled")
```

I think this is important, as we will need to know what happens when something is not working :-)