* Remove serde(skip_serializing_if = ...) attribute from chunk response field
* Make the start response field required by changing its type from
Option<String> to String
`rooms` and `not_rooms` contained `String` and `RoomId` respectively and
the types have been change to use always `RoomId` as it's more concrete
and is more consistent with the other fields (like `senders` and
`not_senders`).
