fix: fixes according to PR comments

- Reset previous implementation of InMemoryExampleStorage
- Implement default (empty) behaviour of on_piece_completed in trait
  itself
- Now passing a ValidPieceIndex in on_piece_completed

crates/librqbit/examples/custom_storage.rs

@@ -57,10 +57,6 @@ impl TorrentStorage for CustomStorage {
     fn init(&mut self, _meta: &librqbit::ManagedTorrentShared) -> anyhow::Result<()> {
         anyhow::bail!("not implemented")
     }
-
-    fn on_piece_completed(&self, _file_id: usize, _offset: u64) -> anyhow::Result<()> {
-        anyhow::bail!("not implemented")
-    }
 }
 
 #[tokio::main]