Fix and test sequences w/ executemany in pre-exec scenarios

Fix and test sequences w/ executemany in pre-exec scenarios

Fixed issue where an engine that had ``implicit_returning`` set to False
would fail to function when PostgreSQL's "fast insertmany" feature were
used in conjunction with a ``Sequence``, as well as if any kind of
"executemany" with "return_defaults()" were used in conjunction with a
``Sequence``. Note that PostgreSQL "fast insertmany" uses "RETURNING" by
definition, when the SQL statement is passed to the driver; overall, the
``implicit_returning`` flag is legacy and has no real use in modern
SQLAlchemy, and will be deprecated in a separate change.

Fixes: #6963
Change-Id: Id8e3dd50a21b9124f338067b0fdb57b8f608dca8