My HS has accumulated thousands of unreferenced state groups

This issue has been migrated from [#3364](https://github.com/matrix-org/synapse/issues/3364).

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... which are filling up my disk :(

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To check if you are also affected, run this query:

```sql
select count(*) from state_groups sg
left join event_to_state_groups esg on esg.state_group=sg.id
left join state_group_edges e on e.prev_state_group=sg.id
where esg.state_group is null and e.prev_state_group is null;
```

if you see numbers in the thousands, then it is this issue. Otherwise, you're not affected by this issue.