Create WPCOM-renewal-order-items.php

* Uses woocommerce_subscriptions_renewal_order_items hook.
* Saves the new product id in the order_item. Saves the original SKU
* in postmeta meta_value '_original_sku'. Saves the new SKU in renewal order
* meta_value '_sku' and updates the original order meta_value '_sku' to
* the renewal SKU.
*
* Reasoning: Woocommerce seems to get SKUs only from the original order
* meta_value '_sku', so we have to change that for each renewal order.
* Also, the fulfillment house may need to get the SKU from the renewal order,
* and it is a good idea to have a record of the original SKU.