Overhaul stats: Emit events from the `tracker-core` package

[josecelano]

Relates to: #1358

[josecelano]

This might not be needed after all. It seems the tracker-core needs to listen to the torrent-repository events but I don't see the need to emit events yet. I'm closing it, I will re-open it if we need them.

See #1437 (comment)