and the do_connect event.  this will be very helpful; we have to
do the hacky "check the function" thing like we do for column defaults
in order to maintain the old signature for creator() as well.
- add new soft invalidate feature, assign the whole "control invalidation"
job to #3379

:paramref:`.Pool.reset_on_return` parameter as a synonym for ``None``,
so that string values can be used for all settings, allowing
.ini file utilities like :func:`.engine_from_config` to be usable
without issue.
fixes #3375

:meth:`.Connection.invalidate` method, or an invalidation due
to a database disconnect, would fail if the
``isolation_level`` parameter had been used with
:meth:`.Connection.execution_options`; the "finalizer" that resets
the isolation level would be called on the no longer opened connection.
fixes #3302