Limit number of queued events

We received some reports of a high number of duplicate events from a single client. Even though we fixed the underlying bug that caused those events a long time ago, they keep coming up, and we think the app might have them queued and it keeps trying to send them all.

We should avoid this scenario by:
- Limiting the number of queued events (500?)
- Have queued events expire after a while (1 week?)