pgsql: fix items order: Due to a misinterpretation, items where returned in wrong order when RSM was used. Pubsub vanilla `max_items` is reversing the items order to get last items, but when this attribute is not used (RSM being used or not), the items must be in chronological order. This patch fixes it, so RSM returns oldest items by defaut, and empty `<before/>` must be used to get last page (and thus most recent items). Items are always finally ordered chronologically inside the returned page (default by `update` time, but this can be modified when a specific order is requested). Reference field is also fixed to use `updated` instead of `item_id` when item are ordered by `update`.