Clarification: I'm sending a REQ with a `#d` set of app IDs to check for updates.

But I can't include one `since` for each app ID in one query, so I have two options: 
 - use the oldest `created_at` of the app ID set - but if one of the apps is quite old, I reap almost no caching benefit
 - send one filter per app - this would work well, but I could have potentially 100s of apps to check which translates into 100s of backend requests for one check

If the relay sent a timestamp, I could use that as the `since` for the next request. The only drawback is it wouldn't catch any back-dated apps, but that is uncommon.