Redesign allowDiskUse attribute of Meta annotation.

[christophstrobl]

This actually a follow up on #4666 to allow users to leverage the MongoDB disk usage defaults when working with the @Meta annotation which defaults allowDiskUse to false. With the current arrangement we cannot simply change the attribute as this would require a type switch (to String maybe as outlined below) and come with a behaviour change if we switch the default.

String allowDiskUse() default "";