establish explicit join transaction modes

The behavior of "joining an external transaction into a Session" has been
revised and improved, allowing explicit control over how the
:class:`_orm.Session` will accommodate an incoming
:class:`_engine.Connection` that already has a transaction and possibly a
savepoint already established. The new parameter
:paramref:`_orm.Session.join_transaction_mode` includes a series of option
values which can accommodate the existing transaction in several ways, most
importantly allowing a :class:`_orm.Session` to operate in a fully
transactional style using savepoints exclusively, while leaving the
externally initiated transaction non-committed and active under all
circumstances, allowing test suites to rollback all changes that take place
within tests.

Additionally, revised the :meth:`_orm.Session.close` method to fully close
out savepoints that may still be present, which also allows the
"external transaction" recipe to proceed without warnings if the
:class:`_orm.Session` did not explicitly end its own SAVEPOINT
transactions.

Fixes: #9015
Change-Id: I31c22ee0fd9372fa0eddfe057e76544aee627107

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