event cache: having any ignored user will aggressively clear all rooms' caches

[bnjbvr]

If the sync returns a state event of the kind IgnoredUserList, then this triggers the observable ignore_user_list_changes in the base client:

matrix-rust-sdk/crates/matrix-sdk-base/src/client.rs

Lines 1294 to 1301 in c8536e9

	if let Some(event) = changes.account_data.get(&GlobalAccountDataEventType::IgnoredUserList)
	{
	match event.deserialize_as::<IgnoredUserListEvent>() {
	Ok(event) => {
	let user_ids: Vec<String> =
	event.content.ignored_users.keys().map(|id| id.to_string()).collect();
	self.ignore_user_list_changes.set(user_ids);

Unfortunately, this will cause a clear of all the rooms in the event cache:

matrix-rust-sdk/crates/matrix-sdk/src/event_cache/mod.rs

Lines 269 to 274 in c8536e9

	while ignore_user_list_stream.next().await.is_some() {
	info!("Received an ignore user list change");
	if let Err(err) = inner.clear_all_rooms().await {
	error!("when clearing room storage after ignore user list change: {err}");
	}
	}

This might not be an issue, if only the initial sync contained the ignored-user-list event, and the event cache has been subscribed to after the first initial sync has been received. But in general, this is a footgun that would be nice to avoid. Things I can imagine to solve this:

• only trigger the observable when the value changed, compared to a previous value we knew about. Meaning we'd compare the new value with the one stored in the database.
• equivalently, maintain a list of ignored users in the event cache, and only clear the room if this list has changed

Likely a blocker for #3280.

cc @Hywan

[bnjbvr]

#4779 includes a patch.