Fix indexing issue and rewrite conflict with Multiwarehouse

@jonpday Can you please review and merge? Modified the rewrite for product_indexer_price_configurable to point to the right class.  Modified the price indexer to extend from multiwarehouse if installed and include the stock_id in the index
@andrew-dwyer I'm not keen on the duplication, but the eval option is worse because it stops you from debugging through it and messes up line numbers in error messages.  Given that the class in question is small, I think this is the better approach.  I also like how you've pulled the two methods that do actually change into a base class so that you're only duplicating a small amount of code.  Overall, I think what you've got is the much better solution!