restore(identifier, etag) {
		return this._getPack()
			.then(pack =>
				pack.get(identifier, etag === null ? null : etag.toString())
			)
			.catch(err => {
				if (err && err.code !== "ENOENT") {
					this.logger.warn(
						`Restoring failed for ${identifier} from pack: ${err}`
					);
					this.logger.debug(err.stack);
				}
			});
	}
@param {string} identifier unique name for the resource
@param {Etag | null} etag etag of the resource
@returns {Promise<Data>} promise to the cached content