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?