Retrieves a path info record for a given path.

def ReadPathInfo(self, client_id, path_type, components, timestamp=None):
    """Retrieves a path info record for a given path."""
    try:
      path_record = self.path_records[(client_id, path_type, components)]
      return path_record.GetPathInfo(timestamp=timestamp)
    except KeyError:
      raise db.UnknownPathError(
          client_id=client_id, path_type=path_type, components=components)