Fixes https://github.com/vector-im/riot-web/issues/14798 (part 2)
This is in two parts itself: The `RoomSublist` needs to break its references to the `RoomListStore`, so it now clones the room arrays. The `Algorithm` is the other part, which is slightly more complicated.
It turns out that we weren't handling splicing as a change in the `ImportanceAlgorithm`, therefore the `Algorithm` wasn't really feeling like it needed to change anything. Further, the `Algorithm` was using the wrong reference to where it should be dumping rooms (`this.cachedRooms` is a getter which returns a different object depending on conditions), so having fixed that we need to ensure that the filtered and sticky maps are also updated when we remove a room. Because we send the new tag through a Timeline update, we'll end up updating the tag later on and don't need to update the filter and sticky collections.
This reduces the update cost of rooms changing, and fixes a bug where when a sublist became filtered it would change the notification count of the sublist.
This does change the expected usage of the state store to ensuring that only one place updates the rooms on the list states, which is currently the room list store. Ideally the state store could listen to the room list store to update itself, however due to a complicated require() loop it is not possible.
This cuts the render time in half (from ~448ms to ~200ms on my account) per received event, as we're no longer re-mounting the entire room list and instead just the section(s) we care about.
