Overhaul stats: decouple events from stats

[josecelano]

To implement the segregated metrics for each server we need to create one event listeners and metrics repository for each service. That means the stats setup function will not work anymore. For example, the stats setup function for the HTTP core is

pub fn factory(
    tracker_usage_statistics: bool,
) -> (
    Option<Box<dyn statistics::event::sender::Sender>>,
    statistics::repository::Repository,
) {
    let mut stats_event_sender: Option<Box<dyn statistics::event::sender::Sender>> = None;

    let mut keeper = statistics::keeper::Keeper::new();

    if tracker_usage_statistics {
        let (sender, _) = broadcast::channel(CHANNEL_CAPACITY);

        let receiver = sender.subscribe();

        stats_event_sender = Some(Box::new(ChannelSender { sender }));

        keeper.run_event_listener(receiver);
    }

    (stats_event_sender, keeper.repository)
}

It creates the channel and subscriber one event listener. we need to separate the channel creation from the listeners subscriptions. My plan is to:

• Move the event enums, sender trait and channel creation to a new "pure" events module.
• Leave the listener subscription in the statistics module.

The new events are "generic" events that can be used for any purpose, not only generating stats.

This will allow later to create one listener and repo per server (socket address).

In this issue, I'm going to only split the modules internally, the statistics::setup::factory(...) function signature will be the same. We can removed it when we introduce the new listeners.