当异步操作启动（如 TCP 服务器接收新连接）或完成（如将数据写入磁盘）时，会调用回调通知用户。
before 回调将被调用 0 到 N 次。
如果异步操作被取消，或者例如，如果 TCP 服务器没有接收到连接，则
before 回调通常会被调用 0 次。
像 TCP 服务器这样的持久异步资源通常会多次调用
When an asynchronous operation is initiated (such as a TCP server receiving a
new connection) or completes (such as writing data to disk) a callback is
called to notify the user. The
before callback is called just before said
callback is executed.
asyncId is the unique identifier assigned to the
resource about to execute the callback.
before callback will be called 0 to N times. The
will typically be called 0 times if the asynchronous operation was cancelled
or, for example, if no connections are received by a TCP server. Persistent
asynchronous resources like a TCP server will typically call the
callback multiple times, while other operations like
fs.open() will call
it only once.