Define interface on consumer-side, to don't depend on confluence repo

[AskAlexSharov]

Dependency on confluence package

• causing dependency cycle

  github.com/anacrolix/torrent@v1.35.1-0.20211104090255-eaeb38b18c6a ->
  github.com/anacrolix/confluence@v1.8.0 -> 
  github.com/anacrolix/dht/v2@v2.10.0 -> 
  github.com/anacrolix/torrent@v1.28.1-0.20210622065255-582f0ccd48a0

• non-functional (it doesn't really use any functionality from confluence)
• I understand that having compile-time interface check is useful - but price looks high

Maybe we can test it on another side? Inside confluence repo (at compile-time or by integration test). Or maybe "debug-writer" package must move to own repo?

[anacrolix]

Thanks!