in callcounts specific to the "expiration" of objects, as in
the "auto expire" feature of :meth:`.Session.commit` and
for :meth:`.Session.expire_all`, as well as in the "cleanup" step
which occurs when object states are garbage collected.
fixes #3307

size of the many per-column objects we're hitting, but somehow the overall memory is
hardly being reduced at all in initial testing

and :meth:`.AttributeEvents.dispose_collection`, which track when
a collection is first associated with an instance and when it is
replaced.  These handlers supersede the :meth:`.collection.linker`
annotation. The old hook remains supported through an event adapter.

sqlalchemy/orm, sqlalchemy/event, sqlalchemy/testing

to get all flake8 passing

when we do a get; we return the None as always but we leave the dict blank
and the loader callable still in place.  The case for this implicit get on a pending object is not
super common and there really should be no change in state at all when this
operation proceeds.   This change is more dramatic as it reverses
a behavior SQLA has had since the first release.
fixes #3061

of work such that loading for related many-to-one objects is slightly
more aggressive, in the case of a graph of self-referential objects
that are to be deleted; the load of related objects is to help
determine the correct order for deletion if passive_deletes is
not set.
- revert the changes to test_delete_unloaded_m2o, these deletes do in fact
need to occur in the order of the two child objects first.