Fix and deprecate min_child_price_* properties

They are from a time when we had one stockrecord per product, and no
strategy classes. They're used for visual purposes only, and were
forgotten when the codebase was updated. They failed loudly, as reported
in #1592.

We really need to either drop them or add a helper method to the
strategy class, but to ensure backwards-compatibility, I've
re-implemented the naive approach of just sorting by price; blindly
ignoring the many shortcomings.

I also added a deprecation notice.
Merge pull request #1594 from django-oscar/issue/1592/min_child_price

Fix and deprecate min_child_price_* properties