Leave the generic management of fields default to the orm as much as
possible.
* Default method is not called unless necessary.
* Default values are correctly post-processed by the orm when necessary.
When default_get is called, the wanted fields are specified through the
fields_list arg. It is useless to fill the values for unwanted fields.
As default_get is called for nearly all records creation, simplifying
the default_get overrides:
* remove potential wrong side-effects of the values
* remove some useless or wrong defaults computations (searches, refs,
...)