storePromise(identifier, etag, data) {
		return new Promise((resolve, reject) => {
			this._cache.store(`${this._name}|${identifier}`, etag, data, err => {
				if (err) {
					reject(err);
				} else {
					resolve();
				}
			});
		});
	}
@template T
@param {string} identifier the cache identifier
@param {Etag | null} etag the etag
@param {T} data the value to store
@returns {Promise<void>} promise signals when the value is stored