10920: Sku => Entity_id relations are fetched inefficiently when inserting attributes values during product import.

[nmalevanec]

Fix for "Sku => Entity_id relations are fetched inefficiently when inserting attributes values during product import"

Description

Replace fetchOne() in loop with getting product ids from protected property _oldSku, which has all necessary information when attributes begin to save, in Magento/CatalogImportExport/Model/Import/Product::_saveProductAttributes() to increase efficiency during product import.

Fixed Issues (if relevant)

1. Sku => Entity_id relations are fetched inefficiently when inserting attributes values during product import #10920: Issue title Sku => Entity_id relations are fetched inefficiently when inserting attributes values during product import

Manual testing scenarios

1. Run import of products from sample csv.
2. Check imported products in admin. Make sure all information was correctly imported from catalog_product.csv(sample csv).
3. Compare performance during import with 2.2-develop branch. For instance you can use : http://devdocs.magento.com/guides/v2.0/config-guide/db-profiler/db-profiler.html . Number of queries and overall time execution should decrease compare to 2.2-develop.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)