SpringBoot Generalize spring.data.jpa.repositories.bootstrap-mode

When spring.data.jpa.repositories.bootstrap-mode is either deferred or lazy, JpaRepositoriesAutoConfiguration customizes the EMF to set an AsyncTaskExecutor which will have the effect of enabling background processing.

The current relationship is a bit odd as this property has an effect on the JPA infrastructure in general, not particularly the repositories. Perhaps we could consider moving this property to spring.jpa.bootstrap-mode? If we do so, the "Lazy" value would be specific to repositories so we may need to rephrase it perhaps.