I don't see how this "only update the last document" trick can work with a priority queue because now the "last" record might get released before other messages and the metadata will be wrong.

Further, surely we really don't need to update this metadata (on the "last" record) when the store is only being used to back a `QueueChannel` (priority or not), so why take the overhead?

It seems to me that a separate `CMS` is really needed.