Overhaul stats: Improve torrent-repository package before adding events

[josecelano]

I've been working on simplifying the torrent-repository package. While working on that, I have identified some things that can be improved or fixed.

I will do it before adding the events. Otherwise, it would be a bigger effort to do it later.

I will group the subtasks into three groups, one for each of the main types in the package:

• PeerList
• TrackedTorrent
• TorrentRepository

Refactoring PeerList

• Rename PeerList to PeerAnnouncementMap?
  • The Peer is the latest peer announcement, not only the IP.
  • It's not implemented with a list, although I chose list as a generic list, not because it's implemented with a "list"
• Use the peer socket address instead of peer ID to avoid peers with duplicate socket addresses.
• Use a cache for seeders and leechers counters to avoid iterating the list.
• Increase test coverage.

Refactoring TrackedTorrent

• Document meets_retaining_policy behavior.
• Consider moving the number_of_downloads_increased counter to PeerList.
• Add unit tests.

Refactoring TorrentRepository

• Remove LockTrackedTorrent trait and return the error to upper layers.
• Add a method for len() or is_empty()
• Fix Concurrency edge case in upsert_peer

Replace:

if let Some(existing_entry) = self.torrents.get(info_hash) {
    // ...
} else {
    let inserted_entry = self.torrents.get_or_insert(...)
    // ...
}

With:

let inserted_entry = self.torrents.get_or_insert_with(*info_hash, || {
    TrackedTorrentHandle::new(TrackedTorrent { ... }.into())
});
let mut torrent_guard = inserted_entry.value().lock_or_panic();
torrent_guard.upsert_peer(peer)

• Optimization: Early get() return in upsert_peer

Replace:

if let Some(existing_entry) = self.torrents.get(info_hash) {
    // mutate entry
} else {
    // insert and mutate
}

With:

let entry = self.torrents.get_or_insert_with(*info_hash, || {
    TrackedTorrentHandle::new(
        TrackedTorrent {
            swarm: PeerList::default(),
            downloaded: opt_persistent_torrent.unwrap_or_default(),
        }
        .into(),
    )
});

let mut guard = entry.value().lock_or_panic();
guard.upsert_peer(peer)

cc @da2ce7