Distance between nodes in node discovery

[ayrat555]

https://github.com/paritytech/parity/blob/master/util/network-devp2p/src/discovery.rs#L236

It looks like the distance between nodes is calculated as a common prefix of each byte. Shouldn't it be just a common binary prefix?

I wrote a test

    #[test]
    fn test_distance() {
        let id1 = H256::from(1_000_000_000);
        let id2 = H256::from(9_000_000_000);

        let distance = Discovery::distance(&id1, &id2);

        assert!(distance == 2);
    }

1_000_000_000 - [0, 0, 1, 1, 1, 0, 1, 1, 1, 0, 0, 1, 1, 0, 1, 0, 1, 1, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0]
9_000_000_000 - [0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 1, 0, 0, 0, 0, 1, 1, 1, 0, 0, 0, 1, 0, 0, 0, 1, 1, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0]

So the common prefix is 2.
Discovery::distance returns 24

[sorpaas]

Might be related to #8589

[sorpaas]

From the literature I can find, Kademlia distance are defined via XOR metric. In Parity Ethereum we used log2(XOR) which should have similar effect. So looks like this function is correct.

Please feel free to re-open this issue if you find out I'm wrong!