- Added new method :meth:`.Session.invalidate`, functions similarly

- Added new method :meth:`.Session.invalidate`, functions similarly
to :meth:`.Session.close`, except also calls
:meth:`.Connection.invalidate`
on all connections, guaranteeing that they will not be returned to
the connection pool.  This is useful in situations e.g. dealing
with gevent timeouts when it is not safe to use the connection further,
even for rollbacks.
references #3258

(cherry picked from commit cf7981f60d485f17465f44c6ff651ae283ade377)