Manages a normal crawler thread.

        When a crawler finished, it loads another one if there are still sites
        to crawl.

def manage_crawler(self):
        """
        Manages a normal crawler thread.

        When a crawler finished, it loads another one if there are still sites
        to crawl.
        """
        index = True
        self.number_of_active_crawlers += 1
        while not self.shutdown and index is not None:
            index = self.crawler_list.get_next_item()
            if index is None:
                self.number_of_active_crawlers -= 1
                break

            self.start_crawler(index)