Replacements and redactions for pinned events aren't working in real-time

[jmartinesp]

When we call PinnedEventsLoader::load_events, this will:

• Fetch the pinned event ids for the room.
• For each of those ids, load the associated event from the event cache if present, otherwise request it from the HS.
• Then it'll check if there are already any events in the event cache that are either a replacement or a redaction of this pinned event, and return it too. This shouldn't be a problem for replacements, since they should be aggregated server-side inside the original event, and neither should it be for redactions, since the server should include the redaction info in the unsigned json contents.

This means for pinned events if we receive some of these related-to-pinned-events events in the sync, the current list of pinned events won't change until it's manually reloaded.

It would be nice to have some way to check whether a new event that's either a replacement or a redaction modifies a pinned event id in a room and broadcast that so we can reload the pinned event timeline as needed.