Update database calls to newer ORM

[wernerkrauss]

calling "canPublish" with Stock extension causes a lot of old, uncaced DB queries that add a huge load to websites if we have a few abandoned carts in ProductStockExtension::getTotalStockInCarts().

@forsdahl has some more modern code for that method.

In general it'd be good to update this module for more modern and faster ORM and maybe implement some caching mechanism for the current product's stock.