using dedicated cache context for garner binding

cc @fancyremarker What do you think about something like this?  This gives us a specific place to define cache keys based off the user's current organization + session.  This does not completely solve the problem as you can now just as easily have a stale cache, but this will at least separate caches by `current_organization`.  Ideally we need a way to invalidate caches or to destroy keys based off prefixes.
I haven't tested that it actually expires, I just noticed that the `global_cache_options` are overridden inside garner.

https://github.com/artsy/garner/blob/1292b59a961e7f6dbfc9580b7cdb0a6a5956afc4/lib/garner/cache/identity.rb#L14