ActiveDispatch::Sessions::CacheStore

I've created a patch to add a new session store that is backed by
ActiveSupport::Cache::Store. It has the same functionality as the
existing MemCacheStore except it is far more flexible in that it can
be backed by any ActiveSupport::Cache::Store (i.e. DalliStore,
RedisStore, MongoStore, etc).

Lighthouse ticket:

https://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/6675