Adds Promises to getOperatorList

During active scrolling only one page can be parsed. This PR:
- For future, allows suspend parsing at any moment of getOperatorList or getTextContent;
- Gives 20ms time slot for those operations.

This allows parsing and display several pages at once (see https://gist.github.com/yurydelendik/513e6d9592fc4fd3a520)

This might help change people's perception of the performance when they actively scrolling.