[REF] registry: always perform registry/cache signaling at the end of request

Problem: the update of custom models/fields is not fully transactional, and may
potentially lead to an inconsistent database.  An other problem is creating two
custom fields by writing on a model: if the second one fails, the first one has
been committed without notice.  Retrying the request will give an unexpected
error (duplicate field name).

Solution: never commit in the middle of a request.  If the changes have an
impact on the registry, then mark it as invalid (with a new flag), and signal
registry invalidation after everything has been committed.  If the request
fails, reset the registry.  Both registry and cache invalidation are handled
the same way.