使用push()时,Firebase会创建一个唯一的ID或密钥,例如-KKm9iRSax-scGn7m3Lb.
根据文件：
The push() method generates a unique ID every time a new child is
added to the specified Firebase reference.
和：
The unique ID generated by push() is based on a timestamp,so list
items are automatically ordered chronologically.
此ID或密钥对整个数据库是唯一的还是仅对引用(如/ users)？
