- clarified LoaderStrategy implementations, centralized deferred column loading

- clarified LoaderStrategy implementations, centralized deferred column loading
into DeferredColumnLoader (i.e. deferred polymorphic loader)
- added generic deferred_load(instance, props) method, will set up "deferred" or "lazy"
loads across a set of properties.
- mapper post-fetch now uses all deferreds, no more post-selects inside a flush() [ticket:652]