Deletes all requests and responses for a given flow from the database.

def DeleteAllFlowRequestsAndResponses(self, client_id, flow_id):
    """Deletes all requests and responses for a given flow from the database."""
    flow_key = (client_id, flow_id)
    try:
      self.flows[flow_key]
    except KeyError:
      raise db.UnknownFlowError(client_id, flow_id)

    try:
      del self.flow_requests[flow_key]
    except KeyError:
      pass

    try:
      del self.flow_responses[flow_key]
    except KeyError:
      pass