This field is no longer read from, so we should stop populating it. Once we're
happy that this doesn't break everything, and a rollback is unlikely, we can
think about dropping the column.
The API is now under
/groups/$group_id/setting/m.join_policy
and expects a JSON blob of the shape
```json
{
"m.join_policy": {
"type": "invite"
}
}
```
where "invite" could alternatively be "open".
We're up to schema v47 on develop now, so this will have to go in there to have
an effect.
This might cause an error if somebody has already run it in the v46 guise, and
runs it again in the v47 guise, because it will cause a duplicate entry in the
bbackground_updates table. On the other hand, the entry is removed once it is
complete, and it is unlikely that anyone other than matrix.org has run it on
v46. The update itself is harmless to re-run because it deliberately copes with
the index already existing.
* Split state group persist into seperate storage func
* Add per database engine code for state group id gen
* Move store_state_group to StateReadStore
This allows other workers to use it, and so resolve state.
* Hook up store_state_group
* Fix tests
* Rename _store_mult_state_groups_txn
* Rename StateGroupReadStore
* Remove redundant _have_persisted_state_group_txn
* Update comments
* Comment compute_event_context
* Set start val for state_group_id_seq
... otherwise we try to recreate old state groups
* Update comments
* Don't store state for outliers
* Update comment
* Update docstring as state groups are ints
Create the url_cache index on local_media_repository as a background update, so
that we can detect whether we are on sqlite or not and create a partial or
complete index accordingly.
To avoid running the cleanup job before we have built the index, add a bailout
which will defer the cleanup if the bg updates are still running.
Fixes https://github.com/matrix-org/synapse/issues/2572.
