message: Replace last_entry with last_key_value
diff: @@ -56,7 +56,7 @@ impl FileHandler {
  let new_fd = candidate_new_fd.unwrap_or_else(|| {
  // find_map ran out of BTreeMap entries before finding a free fd, use one plus the
  // maximum fd in the map
  - self.handles.last_entry().map(|entry| entry.key().checked_add(1).unwrap()).unwrap_or(min_fd)
  + self.handles.last_key_value().map(|(fd, _)| fd.checked_add(1).unwrap()).unwrap_or(min_fd)
  });
  self.handles.insert(new_fd, file_handle).unwrap_none();
  