新增于: v10.0.0
如果此 Http2Session 实例仍在连接，则将是 true，在触发 connect 事件和/或调用 http2.connect 回调之前将设置为 false。
Added in: v10.0.0
Will be true if this Http2Session instance is still connecting, will be set
to false before emitting connect event and/or calling the http2.connect
callback.