- :func:`.attributes.get_history()` when used with a scalar column-mapped

- :func:`.attributes.get_history()` when used with a scalar column-mapped
attribute will now honor the "passive" flag
passed to it; as this defaults to ``PASSIVE_OFF``, the function will
by default query the database if the value is not present.
This is a behavioral change vs. 0.8.  [ticket:2787]
- Added new method :meth:`.AttributeState.load_history`, works like
:attr:`.AttributeState.history` but also fires loader callables.