Dont return outer transaction for _subtrans flag

Dont return outer transaction for _subtrans flag

Fixed critical regression where the :class:`_orm.Session` could fail to
"autobegin" a new transaction when a flush occurred without an existing
transaction in place, implicitly placing the :class:`_orm.Session` into
legacy autocommit mode which commit the transaction. The
:class:`_orm.Session` now has a check that will prevent this condition from
occurring, in addition to repairing the flush issue.

Additionally, scaled back part of the change made as part of :ticket:`5226`
which can run autoflush during an unexpire operation, to not actually
do this in the case of a :class:`_orm.Session` using legacy
:paramref:`_orm.Session.autocommit` mode, as this incurs a commit within
a refresh operation.

Fixes: #6233
Change-Id: Ia980e62a090e39e3e2a7fb77c95832ae784cc9a5