socket.connecting
如果为 true 则 socket.connect(options[, connectListener]) 被调用但还未结束。
它将保持为真，直到 socket 连接，然后设置为 false 并触发 'connect' 事件。
注意，socket.connect(options[, connectListener]) 的回调是 '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.