Fix for Issue #278 (Import -> Stores with large amount of Configurable Products)

This fix is for Stores with large amounts of Products and Configurable Attributes.

_loadSkuSuperAttributeValues() is given parameters and a foreach loop creates the productIds needed for the current bunch.

The conversion From Sku to Id is duplicated later in save(), but I felt it is best to do the conversion here then let the DB do the work. (Cheap Hash Lookups.)

This fix is based off of a module rewrite for EE 1.12 and has not been tested on Mage2. It should work though, That Module successfully imported 700,000 Simple Products and many (80K~ish) Configurable Products. Most of the Configurable Products having 3 super_attributes.

It has also been tested by @barbazul on CE 1.7.0.2. And it worked.

The changes could be slightly cleaner. To do so I would have to had made a bigger change to the structure of the Module. This version of the fix is conceptually cleaner.

Whatever tests you currently have for testing Import -> Products will work to cover this Fix also.

Signed-off-by: Steven Hoffman <s.hoffman@harapartners.com>