VID-2138 Update VideosModule::getWikiRelatedVideosTopics to use AsyncCache

As part of our ongoing performance monitoring, we identified VideosModule::getWikiRelatedVideosTopics as a candidate for improving. After some initial investigation, we determined that implementing the caching for that method using the new AsynCache library would increase our cache hits and help bring down the average response time. This ticket is implementing that change.

Jira ticket: https://wikia-inc.atlassian.net/browse/VID-2138
$numRequired is never anything other than the default. Let's just use that and clean this up until we do need to return a different number of videos based on the request from the client.