Fix generation of sitemap file for large product quantities.

[vovayatsyuk]

http://axiscommerce.com/forum/%D0%B3%D0%B5%D0%BD%D0%B5%D1%80%D0%B0%D1%86%D0%B8%D1%8F-sitemap-%D0%B2%D0%B8%D1%81%D0%BD%D0%B5%D1%82-%D0%BF%D1%80%D0%B8-%D0%B1%D0%BE%D0%BB%D1%8C%D1%88%D0%BE%D0%BC-%D0%BA%D0%BE%D0%BB%D0%B8%D1%87%D0%B5%D1%81%D1%82%D0%B2%D0%B5-%D1%82%D0%BE%D0%B2%D0%B0%D1%80%D0%BE%D0%B2

The file should be written with iterations to safe the memory and execution time limit, similar to search indexes generation.