Set max gap to 32 for low delay connections

[huitema]

When testing performance, we observed that the data rate of long duration connections was lower than short connections. For example, instead of seeing data rates of 3 Gbps with Cubic, we were seeing data rates of 1.5 Mbps.

Analyzing the data showed that picoquic was setting the "max ACK GAP" to very large values, and that ACKs were only sent when the max ACK delay expired, i.e., after 1ms. Capping the "max ack gap" to never exceed 32 packets has a radical effect on the observed Cubic performance. Instead of 1.5 Gbps +- 8%, I now see 3.36 Gbps +- 1%!

We don't see the same improvement for BBR. We see some: the dark blue line (BBR with 32 cap on ack gap) is a bit above the light blue one. Some bad effects are gone: there are no spurious packet losses any more. But there is still work to do to improve that behavior.