fix(conn): cap connections to minimum needed

[tuliperis]

This PR fixes #864

Instead of c = min(n, c), we take into account p: c = min(ceil(n / p), c)

Manually tested:

cargo run --features http3 -- --insecure --http-version {1.1,2,3} https://localhost/ -n 3 -q 3 -p 2

2 per protocol

3 per protocol

cargo run --features http3 -- --insecure --http-version {1.1,2,3} https://localhost/ -n 3 -q 3

1 connection

cargo run --features http3 -- --insecure --http-version 2 https://localhost/ -n 3 -c 50 -p 3