[IMP] ir_model: enable the creation of transient custom models

Rename the field 'osv_memory' on 'ir.model' into 'transient', and make it a
regular field instead of a computed one.  When instantiating a custom model,
use its value to make transient models.

This commit alone will make your server crash on an existing database.  A small
database migration is needed: simply add a boolean column `transient` (with
default value false) in table `ir_model`.