*: refactor edit_order of Base._edit

Instead of overriding the method and adjusting edit order in some class,
define "_default_edit_order" as a class value and use it in Base._edit
as default value when caller do not explicitly pass edit_order.

This was made to keep the default edit order consistent with the order
of edits on python2 for properties where the edit order matters.

This affects mostly scripts, when for example in a script we do:

  delivery.edit(start_date=d, stop_date=d)

on python2 without PYTHONHASHSEED, stop_date is also set, so we keep
this behavior (that is assumed by some tests).

We also change the order of edit for other properties not constraint
by edit_order to edit them in alphabetic order, to have a constant
deterministic behavior.

Co-authored-by: Kazuhiko SHIOZAKI <kazuhiko@nexedi.com>