[ADD] validate_fk_constraints function

[thomaspaulb]

This is useful in cases where you need to delete a lot of records, and you speed the deletion up by disabling FK triggers, but you want to validate integrity after deletion.

Sample use:

env.cr.execute('alter table res_partner disable trigger all')
env.cr.execute(delete from res_partner where id = 738152')
validate_fk_constraints(env.cr, 'res_partner', 'id')
# of course this will fail, so then you can iterate until referential integrity is achieved again