timeline/event-cache: try to re-decrypt bundled events

[bnjbvr]

When we receive an event that contains bundled aggregations (like a replace (edit)), the bundled events could be UTDs. As a matter of fact, we should retry decrypting them later, if we can, which we don't do at the moment.

Since the timeline will incorporate bundled aggregations into a message, including an edit, this may result in an edit not showing up, if the edit event was a UTD at the time of receiving the bundled event. This should happen infrequently, though, so I deem this is low-priority.