Refactor deferred model implementation

[Anssi Kääriäinen]

There are a couple of places in the deferred model implementation which needed cleanup.

• in models/query_utils.py:
  1. the attribute loaded is dead code.
  2. the weak_ref to model isn't needed.
  3. improve the handling of inheritance primary key cases (if parent_ptr_id is already fetched, then assessing pk should not generate any database queries, as parent_ptr_id already contains the value).
• in models/base.py:
  1. Model.__reduce__ contained a dead variable, and stupid logic using that dead variable.
• a minor cleanup to tests/modeltests/field_subclassing/tests.py to spot errors earlier if they happen

The work is tracked in branch ​defer_inheritance_pk, I will create a pull request soon.

[Anssi Kääriäinen]

Pull request: ​https://github.com/django/django/pull/80

[Anssi Kääriäinen <akaariai@…>]

In [a8a81aae20a81e012fddc24f3ede556501af64a2]:

Fixed #18343 -- Cleaned up deferred model implementation

Generic cleanup and dead code removal in deferred model field loading
and model.reduce().

Also fixed an issue where if an inherited model with a parent field
chain parent_ptr_id -> id would be deferred loaded, then accessing
the id field caused caused a database query, even if the id field's
value is already loaded in the parent_ptr_id field.