#22 Compute median of server response deltas instead of average

[lukevalenta]

To test:

./getroughtime -config config.json
Cloudflare-Roughtime: 2024-03-12 10:54:47 -0400 EDT ±1s (in 48ms)
Int08-Roughtime: 2024-03-12 10:54:47.247468 -0400 EDT ±1s (in 29ms)
skipped Google-Roughtime: no reply
Cloudflare-Roughtime-2: 2024-03-12 10:54:50 -0400 EDT ±1s (in 50ms)
Delta: -171ms

cat config.json
{
    "servers": [
        {
            "name": "Cloudflare-Roughtime",
            "version": "IETF-Roughtime",
            "publicKeyType": "ed25519",
            "publicKey": "gD63hSj3ScS+wuOeGrubXlq35N1c5Lby/S+T7MNTjxo=",
            "addresses": [
                    {
                        "protocol": "udp4",
                        "address": "roughtime.cloudflare.com:2002"
                    }
            ]
        },
        {
            "name": "Int08-Roughtime",
            "version": "Google-Roughtime",
            "publicKeyType": "ed25519",
            "publicKey": "AW5uAoTSTDfG5NfY1bTh08GUnOqlRb+HVhbJ3ODJvsE=",
            "addresses": [
                    {
                        "protocol": "udp4",
                        "address": "roughtime.int08h.com:2002"
                    }
            ]
        },
        {
            "name": "Google-Roughtime",
            "version": "Google-Roughtime",
            "publicKeyType": "ed25519",
            "publicKey": "etPaaIxcBMY1oUeGpwvPMCJMwlRVNxv51KK/tktoJTQ=",
            "addresses": [
                    {
                        "protocol": "udp4",
                        "address": "roughtime.sandbox.google.com:2002"
                    }
            ]
        },
        {
            "name": "Cloudflare-Roughtime-2",
            "version": "IETF-Roughtime",
            "publicKeyType": "ed25519",
            "publicKey": "0GD7c3yP8xEc4Zl2zeuN2SlLvDVVocjsPSL8/Rl/7zg=",
            "addresses": [
                    {
                        "protocol": "udp4",
                        "address": "roughtime.cloudflare.com:2003"
                    }
            ]
        }
    ]
}