Open files all at once if necessary.

def _OpenFilesForRead(self, metadata_value_pairs, token):
    """Open files all at once if necessary."""
    aff4_paths = [
        result.AFF4Path(metadata.client_urn)
        for metadata, result in metadata_value_pairs
    ]
    fds = aff4.FACTORY.MultiOpen(aff4_paths, mode="r", token=token)
    fds_dict = dict([(fd.urn, fd) for fd in fds])
    return fds_dict