Store refactor: make it easier to test stores (#9290)

* refactor: convert RoomViewStore from flux Store to standard EventEmitter

Parts of a series of experimental changes to improve the design of stores.

* Use a gen5 store for RoomViewStore for now due to lock handling

* Revert "Use a gen5 store for RoomViewStore for now due to lock handling"

This reverts commit 1076af071d997d87b8ae0b0dcddfd1ae428665af.

* Add untilEmission and tweak untilDispatch; use it in RoomViewStore

* Add more RVS tests; remove custom room ID listener code and use EventEmitter

* Better comments

* Null guard `dis` as tests mock out `defaultDispatcher`

* Additional tests

test/components/structures/RoomView-test.tsx

@@ -42,6 +42,7 @@ import { RightPanelPhases } from "../../../src/stores/right-panel/RightPanelStor
 import { LocalRoom, LocalRoomState } from "../../../src/models/LocalRoom";
 import { DirectoryMember } from "../../../src/utils/direct-messages";
 import { createDmLocalRoom } from "../../../src/utils/dm/createDmLocalRoom";
+import { UPDATE_EVENT } from "../../../src/stores/AsyncStore";
 
 const RoomView = wrapInMatrixClientContext(_RoomView);
 
@@ -74,12 +75,13 @@ describe("RoomView", () => {
     const mountRoomView = async (): Promise<ReactWrapper> => {
         if (RoomViewStore.instance.getRoomId() !== room.roomId) {
             const switchedRoom = new Promise<void>(resolve => {
-                const subscription = RoomViewStore.instance.addListener(() => {
+                const subFn = () => {
                     if (RoomViewStore.instance.getRoomId()) {
-                        subscription.remove();
+                        RoomViewStore.instance.off(UPDATE_EVENT, subFn);
                         resolve();
                     }
-                });
+                };
+                RoomViewStore.instance.on(UPDATE_EVENT, subFn);
             });
 
             defaultDispatcher.dispatch<ViewRoomPayload>({