quic: when staying inside callback

I am again debugging.

I just shot myself in the found,
as the client code in ondatagram stayed forever in the ngtcp2 callback scope and prevented sending data out (SendPendingData blocks it).

I wonder if it is a design problem, as people can stay in js code side of a ngtcp2 callback forever and are interfering with SendPendingData.
Here, I could mitigate it (it was actually sending data from js side and closing), but may be it is an design issue.

@jasnell @pimterry
Your opinions?

首先跟踪 ondatagram 路径，以及它与 ngtcp2 回调作用域和 SendPendingData 的交互。该 issue 未指定文件或测试，也未定义完成标准；首先确定回调执行是否会阻塞发送，以及预期的设计变更是什么（如果有）。