This PR changes the handling of notifications
- removes the `StreamEvent` and `ReadUpdate` stream
- listens on the `OutputRoomEvent` stream in the UserAPI to inform the
SyncAPI about unread notifications
- listens on the `OutputReceiptEvent` stream in the UserAPI to set
receipts/update notifications
- sets the `read_markers` directly from within the internal UserAPI
Co-authored-by: Neil Alexander <neilalexander@users.noreply.github.com>
