DoH: Track the amount of in flight outgoing data

DoH: Track the amount of in flight outgoing data

Previously we would limit the amount of incoming data to process based
solely on the presence of not completed send requests. That worked,
however, it was found to severely degrade performance in certain
cases, as was revealed during extended testing.

Now we switch to keeping track of how much data is in flight (or ready
to be in flight) and limit the amount of processed incoming data when
the amount of in flight data surpasses the given threshold, similarly
to like we do in other transports.