Requires https://github.com/matrix-org/gomatrixserverlib/pull/376
This has numerous upsides:
- Less type casting to `*Event` is required.
- Making Dendrite work with `PDU` interfaces means we can swap out Event
impls more easily.
- Tests which represent weird event shapes are easier to write.
Part of a series of refactors on GMSL.
Adds tests for `QueryRestrictedJoinAllowed`, `IsServerAllowed` and
`PerformRoomUpgrade`. Refactors the `QueryRoomVersionForRoom` method to
accept a string and return a `gmsl.RoomVersion` instead of req/resp
structs.
Adds some more caching for `GetStateEvent`
This should also fix#2912 by ignoring state events belonging to other
users.
