Sometimes bytes_in_flight drops to 0 for longer period of time

[mb]

Another observation from #1472: bytes_in_flight stays at 0 for ~125ms for the first two times, for 50ms the third time.

The graph isn't that informative without interaction. All the packet_sent events are overlayed with the bytes_in_flight.
You can still see the three drop of bytes_in_flight to 0 for no apparent reason.

Could be scheduling, but we receive acks and try to send out data after receiving acks, so improbable. It might be flow control, but I doubt that, will verify that.

[mb]

Could be caused by:

• https://bugzilla.mozilla.org/show_bug.cgi?id=1862629
• https://bugzilla.mozilla.org/show_bug.cgi?id=1862630