quic: when staying inside callback

[martenrichter]

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?