[FIX] core: correct cron thread registry iteration

Fixes #3387
[FIX] website_sale: don't display twice some products

In list view, the 10 last products of a page were the 10 first products of the next page.
Fixes #3373
[FIX] report: fix report HTML editor scrollbar in Firefox
[FIX] purchase: fixed product_uom and price_unit update on product_id change on purchase.order_line form view
[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
[IMP] all modules: use _fields instead of _all_columns
[IMP] edi: use _fields or fields_get() instead of _all_columns
[IMP] mail, website, ir_translation: use _fields instead of _all_columns
[IMP] convert convert.py to _fields
[IMP] add base_field and simplify ir_translation
[IMP] adapt fields.py
[IMP] adapt expression.py
[IMP] adapt models.py