Update of Magento stock quantity: reduced the memory footprint and improved...

performance by using read() instead of browse() and proceeding by steps of 1000
records at a time.

Bug report: https://code.launchpad.net/bugs/1330450
Original MP: https://code.launchpad.net/~camptocamp/openerp-connector-magento/7.0-update-stock-1330450/+merge/223393
this value is not passed by the call to the method on line 157, so I don't understand how an extended  `_magento_qty` method can use additional fields.