Create spring-boot-persistence module to house general persistence-related code and properties

[wilkinsona]

Framework's PersistenceExceptionTranslationPostProcessor is in its spring-tx module in the org.springframework.dao.annotation package. In the restructured 4.0.x branch, auto-configuration for it is now in spring-boot-tx in the org.springframework.boot.transaction.autoconfigure package. The auto-configuration provides a single property, spring.dao.exceptiontranslation.enabled.

[snicoll]

@wilkinsona and I were brainstorming and he suggested to move @EntityScan and friends so that the new spring-boot-persistence doesn't only gather PersistenceExceptionTranslationPostProcessor. I've tried a few options. One in particular was to move @EntityScan outside of the autoconfigure package. This was made possible by breaking the link to AutoConfigurationPackages with a strategy interface. All things considered it's quite a bit of indirection for little benefit.

If we ever want to come back to that idea, a prototype is available in snicoll@ef08392