Store: add onFetch() callback from queryStoreAndInitItems().

This callback is intended to be overridden by classes like `Pageable` that want to process the collection bit-by-bit (one `.range()` at a time) rather than processing the whole thing at once.

(originally mentioned in #165)

I'm not sure on the naming.   Maybe processResults() or something is better

Also, perhaps the postProcessStore() method is no longer necessary as subclasses can just override onFetch().