PLATFORM-615: Use WikiaDataAccess when getting list of hidden categories in DataFeedProvider

Use WikiaDataAccess to limit number of concurrent processes generating the hidden categories list. WikiaDataAccess returns stalled data for up to another cache TTL if some other process is generating 
the fresh version of data

https://wikia-inc.atlassian.net/browse/PLATFORM-615

/cc @macbre @owend 
AFAIR this memcache key will be global - you still need to call ``wfMemcKey``