`/messages` takes ages to back-paginate through private history visibility

This issue has been migrated from [#13128](https://github.com/matrix-org/synapse/issues/13128).

---

If you join a room with `joined` history visibility, then your client will attempt to backfill history. Synapse does the following:

* loads a batch of events from the db
* filters out the ones you can't see (ie, all of them)
* returns the remainder

So the result looks like this:

```json
{
"chunk": [],
"start": "t23104-3060236712_757284974_3318886_1437115222_1456937831_3359724_521254283_4632180264_0",
"end": "t23085-659955750_757284974_3318886_1437115222_1456937831_3359724_521254283_4632180264_0"
}
```

If the history is long, this can go on for ages (like, hours) before we reach the actual start of the room.