agent.reuseSocket(socket, request)
socket<stream.Duplex>request<http.ClientRequest>
当 socket 因保活选项而被持久化后附加到 request 时调用。默认行为是:
【Called when socket is attached to request after being persisted because of
the keep-alive options. Default behavior is to:】
socket.ref(); 此方法可以被特定的 Agent 子类重写。
【This method can be overridden by a particular Agent subclass.】
socket 参数可以是 <net.Socket> 的一个实例，或者是 <stream.Duplex> 的一个子类。
【The socket argument can be an instance of <net.Socket>, a subclass of
<stream.Duplex>.】