[FIX] fields: make field.store=False on old-style function fields

Clarify the semantics of field attributes:
 - field.store is True when the field is actually stored in the database;
 - field.column is the column corresponding to field or None.

The various field definitions correspond to:
 - new-style stored field: field.store and field.column
 - new-style non-stored field: not field.store and not field.column
 - old-style regular field: field.store and field.column
 - old-style function field: not field.store and field.column