Add support for detecting idle apps

#### Is your feature request related to a problem? Please describe

In H2O AI Cloud, we need a reliable way fo telling whether an app is idle or not.
This should include both traffic on the FE (someone is actively looking at the UI) and asynchronous processing.
While the former can be handled automatically in the waved server, the latter needs explicit API in the SDK for the app developer to report if there is any background processing happening or not (we could use proxies like looking at OS-level processes, but that is not ideal)
This idle information then needs to be exposed on a handler for the Cloud service to collect.

#### Describe the solution you'd like

Provide a meta handler (e.g., /_idle) reporting whether the apps registered with the server are idle (or which are idle and which aren't), for some reasonable definition of "idle".

#### Describe alternatives you've considered

We could detect this in the Appstore server when proxying traffic to waved, but that wouldn't cover the backend processing very well.