to have it be weak referencing, use create_session(weak_identity_map=True)
- MySQL detects errors 2006 (server has gone away) and 2014
(commands out of sync) and invalidates the connection on which it occured.
+- MySQL bool type fix: [ticket:307]
- postgres reflection fixes: [ticket:349] [ticket:382]
- added keywords for EXCEPT, INTERSECT, EXCEPT ALL, INTERSECT ALL