def current_multi(id_key_map)
      current_versions = @store.read_multi(*(id_key_map.values))
      Hash[id_key_map.map{ |id, key| [id, current_versions[key]] }]
    end
def current_multi(id_key_map)
      current_versions = @store.read_multi(*(id_key_map.values))
      Hash[id_key_map.map{ |id, key| [id, current_versions[key]] }]
    end
Retrieve the current versions for the given keys
 @param id_key_map is a map with {id => cache_key}
 @return a map with {id => current_version}
 version nil for all keys unknown to the version store