socket.connecting
如果 true，则 socket.connect(options[, connectListener]) 已被调用且尚未完成。
它将保持 true 直到套接字连接，然后将其设置为 false 并触发 'connect' 事件。
请注意，socket.connect(options[, connectListener]) 回调是 'connect' 事件的监听器。
If true,
socket.connect(options[, connectListener]) was
called and has not yet finished. It will stay true until the socket becomes
connected, then it is set to false and the 'connect' event is emitted. Note
that the
socket.connect(options[, connectListener])
callback is a listener for the 'connect' event.