[IMP] fields: reduce the number of attributes on _column instances

This is a memory optimization: instead of setting all attributes on all
instances, set them with their default value on the class, and only set
specific ones on instances.  This reduces the memory footprint of around 14Mb
per registry with modules crm, sale, purchase and stock installed.
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64