Provides a cron task who caches prices for all products and all
pricelists. The goal is to be able to generate a whole catalog of prices
and products for a given customer in a decent time.

Everyday, the cron task will trash the previous day's cache, and rebuild
it from scratch. It means that at any moment, the prices stored in the
cache are those of the current day, and will not be recomputed before
the next day.

However, new prices will be cached in the following cases:

- new product is created
- new pricelist is created
- new pricelist item is created
