Overhaul core Tracker: extract IoC Container

[josecelano]

Parent issue: #1181

I'm working on a big refactor. I'm extracting responsibilities from the core tracker. The Tracker is acting like a kind of IoC container because it instantiates many services. Since I'm extracting those services from the tracker we need a container for them. I'm going to add a IoC container so this code:

#[tokio::main]
async fn main() {
    let (config, tracker, udp_ban_service, stats_event_sender, stats_repository) = bootstrap::app::setup();

    let jobs = app::start(&config, tracker, udp_ban_service, stats_event_sender, stats_repository).await;

    // handle the signals
    tokio::select! {
        _ = tokio::signal::ctrl_c() => {
            tracing::info!("Torrust shutting down ...");

            // Await for all jobs to shutdown
            futures::future::join_all(jobs).await;
            tracing::info!("Torrust successfully shutdown.");
        }
    }
}

can be replaced with something like:

#[tokio::main]
async fn main() {
    let app_container: AppContanier = bootstrap::app::setup();

    let jobs = app::start(&config, app_container).await;

    // handle the signals
    tokio::select! {
        _ = tokio::signal::ctrl_c() => {
            tracing::info!("Torrust shutting down ...");

            // Await for all jobs to shutdown
            futures::future::join_all(jobs).await;
            tracing::info!("Torrust successfully shutdown.");
        }
    }
}

In the Index we called AppData but I think I will call it AppContainer.

There is a crate shaku to handle compilation time dependencies but I don't want to introduce complex dependencies for now. And some dependencies are defined at runtime (database, ..).