- [x] Selective file downloading (mostly done)
  - [x] Proper counting of how much is left, and how much is downloaded

- [x] Send bitfield at the start if I have something
- [x] use the "update_hash" function in piece checking
- [ ] signaling when file is done

- [ ] when we have the whole torrent, there's no point talking to peers that also have the whole torrent

- [ ] per-file stats
- [ ] per-peer stats

- [x] slow peers cause slowness in the end, need the "end of game" algorithm

someday:
- [ ] cancellation from the client-side for the lib (i.e. stop the torrent manager)