Issue 74 - Advice users to set referint-update-delay to 0
    
    Bug Description: Support for asynchronous referential integrity updates
    might be dropped in the future and the referint-update-delay attribute
    might become deprecated. We need to warn the users.
    
    Fix Description: Display a warning when the healthcheck command is
    issued and advise the user to set this attribute to 0.
    
    https://pagure.io/lib389/issue/74
    
    Author: Ilias95
    
    Review by: wibrown (Thanks!)