persist prosody chat messages

If we sometimes need to restart prosody (new release etc) we loose all chat messages in the rooms. Users get back within a few seconds (client auto re-connects).

How can we add a persistent storage?

Discussion https://github.com/jitsi/lib-jitsi-meet/issues/1049

   muc_room_default_persistent = false 

But we also need to consider privacy - ideally when the room is closed and all have left, all messages are cleared. Or add a button?