Overhaul stats events: merge UDP core events with a different IP version

[josecelano]

Change events from this:

bittorrent_udp_tracker_core::statistics::event::Event:

pub enum Event {
    Udp4Connect,
    Udp4Announce,
    Udp4Scrape,
    Udp6Connect,
    Udp6Announce,
    Udp6Scrape,
}

To this:

pub enum Event {
    UdpConnect { context: ConnectionContext },
    UdpAnnounce { context: ConnectionContext },
    UdpScrape { context: ConnectionContext },
}

pub struct ConnectionContext {
    client_socket_addr: SocketAddr,
    server_socket_addr: SocketAddr,
}

This have also be done for HTTP core events.