Update Rails application to use redis-repository-cache

Requirements:

• Add new instance of Gitlab::Redis::RepositoryCache
• Use the new instance of Gitlab::Redis::RepositoryCache
  • To be used in Gitlab::RepositoryCache, Gitlab::RepositoryHashcache, Gitlab::RepositorySetCache
  • Introduced via MultiStore feature flags to migrate the data naturally.
• Add the repository_cache instance in config/README.md