[Monitoring] Do not send requests until the previous one responds

[pickypg]

When on a page with the time picker, you can choose whatever time interval to fetch page updates (defaults to 10s).

However, in the rare situation where the request actually takes longer than the selected interval, the UI will fire the next request before the first one has responded (success or failure). We should stop doing that and block the next request on receiving the first request.

We should hopefully be able to tweak the MonitoringViewBaseController to add this behavior.

Workaround

For those witnessing such behavior on a continuous basis, the best course of action is to pause the automatic fetching and do it on demand until this is corrected.

[pickypg]

/cc @chrisronline based on the discussion we had yesterday.