- Mapped state internals have been reworked to allow for a 50% reduction

- Mapped state internals have been reworked to allow for a 50% reduction
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