Dashboards: Variable of "interval" type causes panels to flicker when refreshing

### What happened? When a dashboard includes a variable of the "interval" type (say, `$window`), any panel that includes a query using that variable flickers when refreshing. More specifically, two network requests to the `render` API are issued in quick succession, with the first one being canceled by the browser, while the second request goes through. It seems that the first, canceled request causes an internal JavaScript error (`runRequest.catchError TypeError: can't access property "message", t.data is null` in `doGraphiteRequest datasource.ts:1112`). In turn, the panel displays the "no data" error very briefly until the second request has returned and the data is rendered again. This causes flickering of the panel, which is visually distracting especially when the dashboard is configured to refresh frequently. This behaviour does not occur when the variable is of a type different from "interval". In that case, a single `render` network request is issued, and the panel doesn't flicker. Therefore, a workaround is to use a "textbox" or "custom" variable type, with the caveat that the "auto" option offered by the "interval" variable type is no longer available. ### What did you expect to happen? A single network request and no panel flickering ("no data" error followed by the graph being redrawn), in line with what happens when variables of types other than "interval" are used. ### Did this work before? It has been an issue in past versions, too (at least since `10.x` but I didn't specifically search). ### How do we reproduce it? 1. Create a dashboard with a Graphite data source and a variable of type "interval" (for example `$window`) 2. Add a panel with a query that includes the variable. A typical query would be `hitcount(perSecond(some.counter), '$window')` but even a nonsensical query that returns no data (for example `alias(*, '$window')` or just `$window`) will cause the issue. The issue also arises when the query is hidden. Removing the variable from the qu…