Do a full QtyReserved and QtyBackordered recalculation in products

Create recalculateInventory(array $product_ids) method, that will:

• get all records from OrderItems table, where given products are being used
• calculate Products.QtyReserved column as SUM(OrderItems.QuantityReserved)
• calculate Products.QtyBackordered column as SUM(IF(OrderItems.BackorderFlag = 1, OrderItems.QuantityReserved - OrderItems.Quantity, 0))
• in all places, where we manipulate with QtyInStock, QtyReserved, QtyBackOrdered columns of product/product order combination we need to use this new method instead

Same applies to product option combinations table.

This cover index would be very helpful to speed up SQL:
ALTER TABLE OrderItems ADD INDEX IX_PRODUCT_INVENTORY_COVER (ProductId, QuantityReserved, BackOrderFlag, Quantity);