Refactor redundant code in util.py

[coderick14]

Extract methods scrape_problem, scrape_contest and handle_batch_requests into a base class and make the specific platform classes inherit from it.

Also, remove dependency on grequests, since it breaks requests depending on the order of import (see issue). The order of imports could be reversed, but the same can be implemented with threading and has the added advantage of reducing the dependency on a third party library.