Reads all message handler requests from the database.

def ReadMessageHandlerRequests(self, cursor=None):
    """Reads all message handler requests from the database."""

    query = ("SELECT UNIX_TIMESTAMP(timestamp), request,"
             "       UNIX_TIMESTAMP(leased_until), leased_by "
             "FROM message_handler_requests "
             "ORDER BY timestamp DESC")

    cursor.execute(query)

    res = []
    for timestamp, request, leased_until, leased_by in cursor.fetchall():
      req = rdf_objects.MessageHandlerRequest.FromSerializedString(request)
      req.timestamp = mysql_utils.TimestampToRDFDatetime(timestamp)
      req.leased_by = leased_by
      req.leased_until = mysql_utils.TimestampToRDFDatetime(leased_until)
      res.append(req)
    return res