Performance Fix for Large Catalogs

Refer to this thread for the full issue. magento/magento2#278

We confirmed this issue Exists in Magento EE 1.13 Where for large catalogs we were experiencing out of memory issues due to this function _loadSkuSuperAttributeValues()

This patch will help those stores with large amounts of configurable products process large imports with hundreds of thousands of rows without hitting this issue as a bottleneck