[IMP] base: add unique constraint on ir.property

There is no unique constraint on the properties to avoid having two
properties for the same field, company and res_id.

Having two properties for the same field, company and res_id, leads to
inconsistencies through the code, as reading a company-dependent field
nondeterministically returns one of the available property value.

So now, before creating a property, we have to check that there is not
already a value for the field, company and res_id and write or create
depending on if it is already exists.

The properties of specific records already satisfy the constraint thanks
to the implementation of company-dependent fields that use the method
`set_multi`.  We added a method `set_default` to set generic properties,
and its implementation does the right thing.  It also simplifies the
code to set such properties, by the way.

closes odoo/odoo#40473

Signed-off-by: Xavier Morel (xmo) <xmo@odoo.com>
Co-authored-by: Raphael Collet <rco@odoo.com>