rust: project 4 benchmarking problems

On slack QuRyu reported some problems completing the project 4 benchmarks: https://tikv-wg.slack.com/archives/CKD9363A8/p1568990445005500

The two main problems:

• They ran into ulimits for open socks. I believe this benchmark describes opening many sockets at once. For simplicity it might be best to pick a number that is lower than the default ulimits. This could be an opportunity to discuss ulimits, which are something that plague networked programs and their test suits, but also this section is already super complicated - adding a ulimit digression would just make it more so.
• They needed to search for an unbound port for the server. This seems to indicate that criterion is running benchmarks in parallel, which is surprising.