VID-2142 Create a way to clear the VideoHandler getVideoList cache

This is the first of two pull requests, split in an effort to not overwhelm reviewers too much.

This PR adds a way to clear the 24 hour VideoHandlerController::getVideoList cache.  Using ```mcache=writeonly```  would achieve similar results but would force cache clearing on several caches we don't care to clear.

As a side effect, this PR also cleans up the ```getVideoList``` method a bit.

https://wikia-inc.atlassian.net/browse/VID-2142
I see, this is not internal. Sounds good!