Reads all flow processing requests from the database.

def ReadFlowProcessingRequests(self, cursor=None):
    """Reads all flow processing requests from the database."""
    query = ("SELECT request, UNIX_TIMESTAMP(timestamp) "
             "FROM flow_processing_requests")
    cursor.execute(query)

    res = []
    for serialized_request, ts in cursor.fetchall():
      req = rdf_flows.FlowProcessingRequest.FromSerializedString(
          serialized_request)
      req.timestamp = mysql_utils.TimestampToRDFDatetime(ts)
      res.append(req)
    return res