Implement `get_query_for_stock` from WC core

[blobaugh]

WooCommerce Core implemented a method to get the current stock quantity for a product. This patch brings that functionality into the feature plugin as well.

Without this patch, customers are unable to checkout with any product that is managing inventory levels.

See core method https://github.com/woocommerce/woocommerce/blob/09d98a6fe059c4a0c5e88a549213a08b56314e85/includes/data-stores/class-wc-product-data-store-cpt.php#L2112

Testing: Before applying this patch

• Create a product that has an inventory stock level
• Add items to the cart
• Proceed through checkout
• Verify checkout fails with a not enough stock notice

After applying this patch

• Add the product with an inventory stock level to cart
• Proceed through checkout
• Verify checkout is successful
• Verify stock count was decremented appropriately

[claudiosanches]

Travis isn't working, but seems like doesn't sound related to this PR, so I'm merging this one now. Thanks!