Adds JpaProperties to bind to spring.jpa.* (making those
properties easier to reason about and visible in the
/configprops endpoint).

Also allows easy configuration of multiple EntityManagerFactories via new
EntityManagerFactoryBuilder. JpaBaseConfiguration has a @Bean of that type
so users can inject it to create new or additional EntityManagerFactories.
This also simplifies the Hibernate autoconfiguration.

Also renames the DataSourceFactory to DataSourceBuilder (since that's what it
is).