function Storage_(prefix, optProperties, optCache) {
  this.prefix_ = prefix;
  this.properties_ = optProperties;
  this.cache_ = optCache;
  this.memory_ = {};
}
Creates a new Storage_ instance, which is used to persist OAuth tokens and
related information.
@param {string} prefix The prefix to use for keys in the properties and
    cache.
@param {PropertiesService.Properties} optProperties The optional properties
    instance to use.
@param {CacheService.Cache} [optCache] The optional cache instance to use.
@constructor