Improve performance of registry loading

Improve performance and consistency of `_setup_fields()` by avoiding recursive calls of method `_inherits_reload()`.  In `_setup_fields()`, first all parent models are set up, then their fields are inspected to determine inherited fields, and their setup is done.  This scheme guarantees that inherited fields are computed once per model. Performance gain is around 10% speedup.

Also, in `_init_manual_fields()`, create fields instead of columns. This avoids an extra loop to convert columns to fields, which was only useful for custom fields.