DE1657 DE1685 Product Feed Optimizations

* Make string usages consistent
* Don't pass storeId to addStoreFilter (it's unnecessary)
* Simplify file opening and writing
* Fixed sending feed file twice
* Added Admin configuration of Product Feed Buffer Size
* Removed ProductsTest.php
* Fix comments in DataTest.php
* Add documentation
DE1509 Previous Commit DE1504 Should Be DE1509