* Request partial joins by default
This is a little sloppy, but we are trying to gain confidence in faster
joins in the upcoming RC.
Admins can still opt out by adding the following to their Synapse
config:
```yaml
experimental:
faster_joins: false
```
We may revert this change before the release proper, depending on how
testing in the wild goes.
* Changelog
* Try to fix the backfill test failures
* Upgrade notes
* Postgres compat?
