store(identifier, etag, data) {
		if (this.readonly) return Promise.resolve();

		return this._getPack().then(pack => {
			pack.set(identifier, etag === null ? null : etag.toString(), data);
		});
	}
@param {string} identifier unique name for the resource
@param {Etag | null} etag etag of the resource
@param {Data} data cached content
@returns {Promise<void>} promise