Timeline needs to refresh when we see a MSC2716 marker event (#2299)

Inform the client that historical messages were imported in the timeline and they should refresh the timeline in order to see the new events.

Companion `matrix-react-sdk` PR: https://github.com/matrix-org/matrix-react-sdk/pull/8354

The `marker` events are being used as state now because this way they can't be lost in a timeline gap. Regardless of when they were sent, we will still have the latest version of the state to compare against. Any time we see our latest state value change for marker events, prompt the user that the timeline needs to refresh.

> In a [sync meeting with @ara4n](https://docs.google.com/document/d/1KCEmpnGr4J-I8EeaVQ8QJZKBDu53ViI7V62y5BzfXr0/edit#bookmark=id.67nio1ka8znc), we came up with the idea to make the `marker` events as state events. When the client sees that the `m.room.marker` state changed to a different event ID, it can throw away all of the timeline and re-fetch as needed.
>
> For homeservers where the [same problem](https://github.com/matrix-org/matrix-doc/pull/2716#discussion_r782499674) can happen, we probably don't want to throw away the whole timeline but it can go up the `unsigned.replaces_state` chain of the `m.room.marker` state events to get them all.
>
> In terms of state performance, there could be thousands of `marker` events in a room but it's no different than room members joining and leaving over and over like an IRC room.
>
> *-- https://github.com/matrix-org/matrix-spec-proposals/pull/2716#discussion_r782629097*


### Why are we just setting `timlineNeedsRefresh` (and [prompting the user](https://github.com/matrix-org/matrix-react-sdk/pull/8354)) instead of automatically refreshing the timeline for the user?

If we refreshed the timeline automatically, someone could cause your Element client to constantly refresh the timeline by just sending marker events over and over. Granted, you probably want to leave a room like this 🤷. Perhaps also some sort of DOS vector since everyone will be refreshing and hitting the server at the exact same time.

In order to avoid the timeline maybe going blank during the refresh, we could re-fetch the new events first, then replace the timeline. But the points above still stand on why we shouldn't.

spec/unit/event-timeline-set.spec.ts

@@ -23,6 +23,7 @@ import {
     MatrixEvent,
     MatrixEventEvent,
     Room,
+    DuplicateStrategy,
 } from '../../src';
 
 describe('EventTimelineSet', () => {
@@ -73,6 +74,76 @@ describe('EventTimelineSet', () => {
         }) as MatrixEvent;
     });
 
+    describe('addLiveEvent', () => {
+        it("Adds event to the live timeline in the timeline set", () => {
+            const liveTimeline = eventTimelineSet.getLiveTimeline();
+            expect(liveTimeline.getEvents().length).toStrictEqual(0);
+            eventTimelineSet.addLiveEvent(messageEvent);
+            expect(liveTimeline.getEvents().length).toStrictEqual(1);
+        });
+
+        it("should replace a timeline event if dupe strategy is 'replace'", () => {
+            const liveTimeline = eventTimelineSet.getLiveTimeline();
+            expect(liveTimeline.getEvents().length).toStrictEqual(0);
+            eventTimelineSet.addLiveEvent(messageEvent, {
+                duplicateStrategy: DuplicateStrategy.Replace,
+            });
+            expect(liveTimeline.getEvents().length).toStrictEqual(1);
+
+            // make a duplicate
+            const duplicateMessageEvent = utils.mkMessage({
+                room: roomId, user: userA, msg: "dupe", event: true,
+            }) as MatrixEvent;
+            duplicateMessageEvent.event.event_id = messageEvent.getId();
+
+            // Adding the duplicate event should replace the `messageEvent`
+            // because it has the same `event_id` and duplicate strategy is
+            // replace.
+            eventTimelineSet.addLiveEvent(duplicateMessageEvent, {
+                duplicateStrategy: DuplicateStrategy.Replace,
+            });
+
+            const eventsInLiveTimeline = liveTimeline.getEvents();
+            expect(eventsInLiveTimeline.length).toStrictEqual(1);
+            expect(eventsInLiveTimeline[0]).toStrictEqual(duplicateMessageEvent);
+        });
+
+        it("Make sure legacy overload passing options directly as parameters still works", () => {
+            expect(() => eventTimelineSet.addLiveEvent(messageEvent, DuplicateStrategy.Replace, false)).not.toThrow();
+            expect(() => eventTimelineSet.addLiveEvent(messageEvent, DuplicateStrategy.Ignore, true)).not.toThrow();
+        });
+    });
+
+    describe('addEventToTimeline', () => {
+        it("Adds event to timeline", () => {
+            const liveTimeline = eventTimelineSet.getLiveTimeline();
+            expect(liveTimeline.getEvents().length).toStrictEqual(0);
+            eventTimelineSet.addEventToTimeline(messageEvent, liveTimeline, {
+                toStartOfTimeline: true,
+            });
+            expect(liveTimeline.getEvents().length).toStrictEqual(1);
+        });
+
+        it("Make sure legacy overload passing options directly as parameters still works", () => {
+            const liveTimeline = eventTimelineSet.getLiveTimeline();
+            expect(() => {
+                eventTimelineSet.addEventToTimeline(
+                    messageEvent,
+                    liveTimeline,
+                    true,
+                );
+            }).not.toThrow();
+            expect(() => {
+                eventTimelineSet.addEventToTimeline(
+                    messageEvent,
+                    liveTimeline,
+                    true,
+                    false,
+                );
+            }).not.toThrow();
+        });
+    });
+
     describe('aggregateRelations', () => {
         describe('with unencrypted events', () => {
             beforeEach(() => {