Replace iperf loadtest with fortio

[markmandel]

https://github.com/googleforgames/quilkin/tree/main/examples/iperf3 seems more tricky than it needs to be.

https://github.com/fortio/fortio seems like a much easier option - let's switch it out!

More context: https://github.com/googleforgames/quilkin/issues/834#issuecomment-1772183595

Server

fortio udp-echo

Quilkin

cargo run --release -- proxy --to 127.0.0.1:8078

Client

fortio load -c 3000 -qps 1000000 udp://127.0.0.1:7777/