purgeStale() {
        let deleted = false;
        for (const i of this.#rindexes({ allowStale: true })) {
            if (this.#isStale(i)) {
                this.delete(this.#keyList[i]);
                deleted = true;
            }
        }
        return deleted;
    }
Delete any stale entries. Returns true if anything was removed,
false otherwise.