Reads blob references for the signed binary with the given id.

def ReadSignedBinaryReferences(
      self, binary_id,
      cursor=None):
    """Reads blob references for the signed binary with the given id."""
    cursor.execute(
        """
      SELECT blob_references, UNIX_TIMESTAMP(timestamp)
      FROM signed_binary_references
      WHERE binary_type = %s AND binary_path_hash = %s
    """, [
        binary_id.binary_type.SerializeToDataStore(),
        mysql_utils.Hash(binary_id.path)
    ])
    row = cursor.fetchone()

    if not row:
      raise db.UnknownSignedBinaryError(binary_id)

    raw_references, timestamp = row
    # TODO(hanuszczak): pytype does not understand overloads, so we have to cast
    # to a non-optional object.
    datetime = cast(rdfvalue.RDFDatetime,
                    mysql_utils.TimestampToRDFDatetime(timestamp))

    references = rdf_objects.BlobReferences.FromSerializedString(raw_references)
    return references, datetime