Using these two values ties requests to a
particular query position within a panel, ensuring
that requests are canceled if:

- Duplicate requests with the same query are sent
- Requests from the same query position (but a
different query) are sent

The last point is important as it identifies
queries by a physical location in the dashboard
instead of with the query expression.  If a
different query from the same position in a panel
is sent, the previous request should be canceled.