rework to expire() to make it smarter. when you expire(), history is immediately...

rework to expire() to make it smarter.  when you expire(), history is immediately removed as well as explicit from dirty/deleted lists.  this also changes uow.rollback_object() to remove from those lists, which is strange that it didnt do that before.  anyway the mapper, when selecting and creating instances, asks the uow if this already identity-mapped instance is expired, and if so refreshes it on the fly, saving the need for the re-_get() operation, if some other query happens to touch upon the expired object.  unit test added to confirm this.