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

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

That's the current way to count all downloads ans expose that mertric.

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).

It's needed a new solution to keep that metric, but in the mean time
this disables that feature, producing

- Non accurate value for downloads when the tracker is restarted
- Increasing indefenitively the number of torrents in memory even if the
  "remove peerless torrents" policy is enabled (becuase this fetaure
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