class BulkCacheFetcher

def partition(object_identifiers)

objects that weren't cached.
from the cache. The second is a list of all of the identifiers for
of {cache_key: object} for all the objects we were able to serve
Splits a list of identifiers into two objects. The first is a hash 
def partition(object_identifiers)
  uncached_identifiers = object_identifiers.dup
  cache_keys = cache_keys(object_identifiers)
  cached_keys_with_objects = @cache.read_multi(*cache_keys)
  cache_keys.each do |cache_key|
    uncached_identifiers.delete(cache_key) if cached_keys_with_objects.key?(cache_key)
  end
  [cached_keys_with_objects, uncached_identifiers]
end