MSC2716: Incremental existing history

Fix https://github.com/matrix-org/matrix-doc/blob/matthew/msc2716/proposals/2716-importing-history-into-existing-rooms.md

Author: Eric Eastwood

clientapi/routing/sendevent.go

@@ -52,6 +52,12 @@ func SendEvent(
 	rsAPI api.RoomserverInternalAPI,
 	txnCache *transactions.Cache,
 ) util.JSONResponse {
+	prevEvent := req.URL.Query().Get("prev_event")
+
+	util.GetLogger(req.Context()).WithFields(logrus.Fields{
+		"prevEvent": prevEvent,
+	}).Info("prevEvent")
+
 	verReq := api.QueryRoomVersionForRoomRequest{RoomID: roomID}
 	verRes := api.QueryRoomVersionForRoomResponse{}
 	if err := rsAPI.QueryRoomVersionForRoom(req.Context(), &verReq, &verRes); err != nil {