Writes a flow object to the database.

def WriteFlowObject(self, flow_obj):
    """Writes a flow object to the database."""
    if flow_obj.client_id not in self.metadatas:
      raise db.UnknownClientError(flow_obj.client_id)

    clone = flow_obj.Copy()
    clone.last_update_time = rdfvalue.RDFDatetime.Now()
    self.flows[(flow_obj.client_id, flow_obj.flow_id)] = clone