[FIX] ir_model_fields: fix and simplify update on custom fields

Avoid "patching" the registry, as this introduces inconsistencies (some field
attributes are lost).  Instead, proceed as follows:
 - update the definition of custom fields in database;
 - clear the corresponding cache on the registry (this was missing);
 - setup the models in registry (this reloads the custom models and fields);
 - update the database schema of the models based on the registry.

This makes the update of custom fields simpler and more robust.