[HG-174] Allow for make export in a single thread

Add configuration options to set up on admin backend whether to make the
export in a single thread or multimple ones. Refactor _exportThread method
to be able to use magento iterators, this way we avoid memory issues when
loading the complete collection in the single thread mode.
Merge branch 'HG-174' into development
Merge branch 'development' into MAGEMEP-155
Merge branch 'MAGEMEP-155' into development
Merge branch 'development' of https://github.com/Flagbit/Magento-MEP into development