state resets still happen in v2 rooms

This issue has been migrated from [#8629](https://github.com/matrix-org/synapse/issues/8629).

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I just upgraded `#synapse:matrix.org` because I got fed up of losing my ops bit. It was a v4 room.

EDIT 2022-08-03 by @DMRobertson: if you suspect you have seen a state reset, please provide:

- the `room_id` of the room with the reset
- the `type` and ` state_key` that suffered a reset
- the event_id of the piece of state after the reset
- ideally, the event_id of the piece of state _before_ the reset
- failing that, a rough time range before and after the reset occurred (including timezone information)

To confirm if this really is a server-side state reset (rather than e.g. matrix-org/matrix-spec#1209), server administrators can run the following queries.

```sql
SELECT *
FROM current_state_delta_stream
WHERE
room_id =''
and type='''
and state_key=''
ORDER BY stream_id asc;

SELECT *
FROM current_state_events
WHERE
room_id =''
and type='''
and state_key='';
```

The last row in the `event_id` columns from these two queries should match. It represents the event which currently describes the relevant piece of state from your server's point of view.