Change bot/Clients.ChangeNick to be on a hook

[Xe]

// ChangeNick changes a client's nickname and updates the ByNick map.
func (c *ClientSet) ChangeNick(client *Client, newnick string) (err error) {
    if _, present := c.ByNick[strings.ToUpper(client.Nick)]; !present {
        Log.Fatalf("Client %s does not exist in Clients.ByNick. We are desynched. Exiting.", client.Nick)
    }

    delete(c.ByNick, client.Nick)

    c.ByNick[strings.ToUpper(newnick)] = client

    return
}

This should be a hook and not manually invoked behavior.