- Support for an optional callback on `publish_entities` to get notified
when the published object are flushed to the db.
- Use `lazy='selectin'` for the entity parent -> children relationship -
it is more efficient and it ensures that all the data the application
needs is loaded upfront.
- `Entity.entity_key` rolled back to `<external_id, plugin>`. The
fallback logic on `<id, plugin>` created more problems than those it
as supposed to solve.
- Added `expire_on_commit=False` to the entities engine session to make
sure that we don't get errors on detached/expired instances.
Fabio Manganiello