Improve room switching in the new room list
For https://github.com/vector-im/riot-web/issues/14034
One of the largest issues with room switching was that we'd regenerate the entire list when the sticky room changes, which is obviously detrimental on larger accounts (and even some medium accounts). To work through this, we simply handle the NewRoom and RoomRemoved causes (used by the sticky room handling) as splices rather than in-place updates.
Overall this leads to a smoother experience as it means we're doing far less calculations and can even opt out of an update if it isn't required, such as a RoomRemoved cause being fired twice - the second one can result in an update not being required, saving render time.
This commit also includes a fix for handling update causes on the sticky room, as the room list loves to print errors when this happens. We don't need to handle any updates because once the sticky room changes it'll get re-added through NewRoom, causing the underlying algorithm to slot it in where needed, effectively handling all the missed updates.
