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 without hitting this issue as a
bottleneck
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