Require all server nodes to be offline/shutdown when initializing meta mirroring

With https://github.com/ThinkParQ/beegfs-core/issues/4385, all server nodes (meta and storage) now need to be restarted after initializing.

To make it less likely to forget this, we should require the nodes to be offline in the first place (except for the two meta nodes from the buddy
 group with the root inode). It is not 100% and it is timer based, but probably not a bad idea.