fix: [#1510] disable torrent stats importation at start

When the tracker starts, if stats persistence is enabled, all torrents that have ever been downloaded are loaded into memory (`Swarms` type) with their download counter.

That's the current way to count all downloads and expose that metric.

However, it does not work with **millions of torrents** (like in the tracker demo) becuase:

- It's too slow.
- It consumes too much memory (all torrents that have ever been downloaded have to be loaded).

A new solution is needed to keep that metric, but in the meantime, this disables that feature, producing these effects:

- Non-accurate value for downloads when the tracker is restarted.
- Increasing indefinitely the number of torrents in memory even if the "remove peerless torrents" policy is enabled (becuase this feature overrides that policy and peerless torrents are kept in memory).

Author: josecelano

src/app.rs

@@ -61,7 +61,12 @@ pub async fn start(config: &Configuration, app_container: &Arc<AppContainer>) ->
 async fn load_data_from_database(config: &Configuration, app_container: &Arc<AppContainer>) {
     load_peer_keys(config, app_container).await;
     load_whitelisted_torrents(config, app_container).await;
-    load_torrents_from_database(config, app_container);
+    // todo: disabled because of performance issues.
+    // The tracker demo has a lot of torrents and loading them all at once is not
+    // efficient. We also load them on demand but the total number of downloads
+    // metric is not accurate because not all torrents are loaded.
+    // See: https://github.com/torrust/torrust-tracker/issues/1510
+    //load_torrents_from_database(config, app_container);
 }
 
 async fn start_jobs(config: &Configuration, app_container: &Arc<AppContainer>) -> JobManager {
@@ -110,6 +115,7 @@ async fn load_whitelisted_torrents(config: &Configuration, app_container: &Arc<A
     }
 }
 
+#[allow(dead_code)]
 fn load_torrents_from_database(config: &Configuration, app_container: &Arc<AppContainer>) {
     if config.core.tracker_policy.persistent_torrent_completed_stat {
         app_container