HTTP/3: traffic-based flood detection.

HTTP/3: traffic-based flood detection.

With this patch, all traffic over HTTP/3 bidi and uni streams is counted in
the h3c->total_bytes field, and payload traffic is counted in the
h3c->payload_bytes field.  As long as total traffic is many times larger than
payload traffic, we consider this to be a flood.

Request header traffic is counted as if all fields are literal.  Response
header traffic is counted as is.