backend, storage(pgsql): creation/update date + serial ids: /!\ this patch updates pgqsl schema /!\ Had to set 2 features in the same patch, to avoid updating 2 times the schema. 1) creation/last modification date: column keeping the date of creation of items is renamed from "date" to "created" the date of last modification of items is saved in the new "updated" column 2) serial ids: this experimental feature allows to have ids in series (i.e. 1, 2, 3, etc.) instead of UUID. This is a convenience feature and there are some drawbacks: - PostgreSQL sequences are used, so gaps can happen (see PostgreSQL documentation for more details) - if somebody create an item with a future id in the series, the series will adapt, which can have undesired effect, and may lead to item fail if several items are created at the same time. For instance if next id in series is "8", and somebody hads already created item "8" and "256", the item will be created with biggest value in items +1 (i.e. 257). if 2 people want to create item in this situation, the second will fail with a conflict error.