VID-2103 Async caching for views to replace cron caching

RFCR @garthwebb @owend 

This is the very first usage of the mighty [async caching](https://github.com/Wikia/app/pull/5045) for caching video view counts on demand rather than via running a cron job every 2 hours to retrieve and cache ALL video views on ALL clustered wikias. See [VID-2103](https://wikia-inc.atlassian.net/browse/VID-2103).

This is done via a new WikiFactory variable `wgEnableAsyncVideoViewCache `. This allows controlled rollout of this solution. Once this is solution proves effective, the variable will be removed.

This also:
- Uses selectDB() in runOnCluster()
- Loads Async cache files in defaultSettings 
Using ```$memKeyBucket``` won't work when ```$asyncCacheEnabled``` is ```true```.  Because ```$memKeyBucket``` is the first two characters of the hashed title, it is not unique (that's why its named 'bucket'; it holds several video titles).  You should probably construct ```$cacheKey``` separately for each case.  For the async case, you'll want to include the full hashed title because its only storing one value per key.