Optimize torrent/peers list reading and writing by breaking up the `tracker.torrents` field into smaller pieces

[mickvandijke]

We were talking about refactoring the torrents map in the Tracker struct to reduce the amount of write locks needed on the entire torrents map. This would theoretically improve torrent announce performance when you have a lot of torrents being updated at the same time.

Refactor:

torrents: RwLock<std::collections::BTreeMap<InfoHash, torrent::Entry>>,

To:

torrents: RwLock<std::collections::BTreeMap<InfoHash, Arc<Mutex<torrent::Entry>>>>,

[josecelano]

Hi @WarmBeer I would suggest taking a look at @Power2All's implementation here and also at this discussion.

[mickvandijke]

Relevant discussion: #504

[mickvandijke]

Superseded by #565