BAC-734 Image Serving results caching

* cache up to 20 images for each article, ``ImageServing::getImages`` will then take an appropriate slice of them
* mark ``ImageServing::getThumbnails`` as deprecated
* use JSON format by default when calling ImageServing controller via wikia.php
* article's metadata (article and namespace ID) are actually stored in memcache (get was there for a while)
* a bit of code cleanup

@Wikia/platform-team