Replace old visible rooms range with subscriptions in the room list. (#3014)

* Add a SubscriptionTracker actor to RoomProxy.

* Request subscriptions when scrolling rooms.

* Stop unsubscribing in preparation for Simplified Sliding Sync.

ElementX/Sources/FlowCoordinators/RoomFlowCoordinator.swift

@@ -680,8 +680,6 @@ class RoomFlowCoordinator: FlowCoordinatorProtocol {
             navigationStackCoordinator.setRootCoordinator(nil, animated: false)
         }
         
-        roomProxy?.unsubscribeFromUpdates()
-        
         timelineController = nil
         
         actionsSubject.send(.finished)