implement autobegin=False option

implement autobegin=False option

Added new parameter :paramref:`_orm.Session.autobegin`, which when set to
``False`` will prevent the :class:`_orm.Session` from beginning a
transaction implicitly. The :meth:`_orm.Session.begin` method must be
called explicitly first in order to proceed with operations, otherwise an
error is raised whenever any operation would otherwise have begun
automatically. This option can be used to create a "safe"
:class:`_orm.Session` that won't implicitly start new transactions.

As part of this change, also added a new status variable
:class:`_orm.SessionTransaction.origin` which may be useful for event
handling code to be aware of the origin of a particular
:class:`_orm.SessionTransaction`.

Fixes: #6928
Change-Id: I246f895c4a475bff352216e5bc74b6a25e6a4ae7