take out accidental PG dialect use here
- Added some new event mechanics for dialect-level events; the initial
implementation allows an event handler to redefine the specific mechanics
by which an arbitrary dialect invokes execute() or executemany() on a
DBAPI cursor.  The new events, at this point semi-public and experimental,
are in support of some upcoming transaction-related extensions.
- rename __multiple__ to __backend__, and apply __backend__ to a large number of tests.
- move out logging tests from test_execute to test_logging
- fix some doctest failures (though some remain, as it's not easy to get doctest
to accept things flexibly), fix #2999