Use PDU not *Event in HeaderedEvent (#3073)

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.

roomserver/producers/roomevent.go

@@ -74,7 +74,7 @@ func (r *RoomEventProducer) ProduceRoomEvents(roomID string, updates []api.Outpu
 			}
 
 			if eventType == "m.room.server_acl" && update.NewRoomEvent.Event.StateKeyEquals("") {
-				ev := update.NewRoomEvent.Event.Event
+				ev := update.NewRoomEvent.Event.PDU
 				defer r.ACLs.OnServerACLUpdate(ev)
 			}
 		}