timeline: aggregated events aren't well ordered

[bnjbvr]

Aggregation events (edits, reactions) aren't well ordered in the timeline: we don't really know how to order them relative to each other. In the general case, it won't be a problem, but it might lead to discrepancies in a few cases:

• when we have a bundled edit, for instance, right now we'll always prefer it to a pending edit. This might be wrong if the pending edit is more up-to-date than the bundled edit (i.e. the related pending edit is after the bundled edit event, in the overall ordering).
• when we already have a pending edit and another comes in, we do some best-effort guessing if the new pending edit overrides the previous one or would be overridden by the previous one.

Ideally, the protocol would give us some perfect ordering over events, so we don't have to do any kind of guessing / rejiggering ourselves, e.g. with something akin to MSC4033. Otherwise, we may be able to use the all_events internal ordering (or the ordering provided by the Event cache, in some future).