This commit reverts partially https://github.com/discourse/discourse/pull/17543.
Service appEvents was not being injected in ScreenTrack. This causes
`this.appEvents.trigger("topic:timings-sent", data);` to fail and the error is
swallowed by the `catch` on the promise.
This caused a regression on plugins that rely on this event to implement other
behaviors.
1. Injecting `appEvents` service into `screen-track` was unnecessary as it's already injected into all services (and was causing an assertion error)
2. Return a promise from `sendNextConsolidatedTiming()` (no need for `await settled()` then)
Instead of relaying on /timings request, we should cache last read post number. That should protect from having incorrect unread counter when going back to topic list.
This additional cache is very temporary as once /timings request is finished, serializer will have a correct result.
Simplified flow is:
1. Store in cache information about last seen post number before /timings request is sent
2. When getting back to topic list compare value of last seen post number returned by /latest request and information in cache. If cache number is higher, than use it instead of information returned by /latest. In addition delete cache item as there is high chance that `/timings` request already finished.
3. Optionally, delete cache when timings request is done and topic list was not yet visited.
Keeping cache reasonably small should not affect performance.
