Implement explicit autobegin step for Session

Implement explicit autobegin step for Session

The :class:`.Session` object no longer initates a
:class:`.SessionTransaction` object immediately upon construction or after
the previous transaction is closed; instead, "autobegin" logic now
initiates the new :class:`.SessionTransaction` on demand when it is next
needed.  Rationale includes to remove reference cycles from a
:class:`.Session` that has been closed out, as well as to remove the
overhead incurred by the creation of :class:`.SessionTransaction` objects
that are often discarded immediately. This change affects the behavior of
the :meth:`.SessionEvents.after_transaction_create` hook in that the event
will be emitted when the :class:`.Session` first requires a
:class:`.SessionTransaction` be present, rather than whenever the
:class:`.Session` were created or the previous :class:`.SessionTransaction`
were closed.   Interactions with the :class:`.Engine` and the database
itself remain unaffected.

Fixes: #5074
Change-Id: I00b656eb5ee03d87104257a214214617aacae16c