[FIX] models: in method onchange(), check for record dirtiness only on *2many fiels

Cascading onchanges can be caused by a related field computed in cache.  This
causes a bug in sale order lines, were setting the uom field forces reading
product fields, which are inherited from product templates.  The inherited
fields are computed as related fields, which marks the product record as dirty.
This subsequently triggers an onchange on the product field, which resets the
uom field!
[FIX] point_of_sale: chrome was dropping the cookie when loading the company logo, resulting in an incorrect logo in multi-db setups.
[FIX] mrp: force empty values for fields that were dropped

This ensures the field values are cleared during
upgrades from previous versions
[FIX] product,purchase,point_of_sale: force empty values for dropped fields

At 96f038a product-related fields were removed due
to an important product.template/product.product
refactoring. As the field values were simply
dropped, they may not be nullified when upgrading an
existing database. Forcing them to False will take
care of it.
[FIX] hr_recruitment: fixed computation and storage of day_open and day_close + assignment of date_open and date_closed.