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/src/file_ops.rs

@@ -215,7 +215,7 @@ impl<'a> FileOps<'a> {
                 format!("error reading {to_read_in_file} bytes, file_id: {file_idx} (\"{name:?}\")")
             })?;
 
-            self.files.on_piece_completed(file_idx, absolute_offset)?;
+            self.files.on_piece_completed(piece_index)?;
 
             piece_remaining_bytes -= to_read_in_file;