cubicweb #352993 Don't stop the server while upgrading [open]

cubicweb-ctl upgrade could do something like:

  1. flag the running application as read-only
  2. copy the database (and whatever is needed, config, etc.)
  3. process the migration on the copy
  4. once the process is over, copy the migrated database
  5. restart the application
done in<not specified>
closed by<not specified>