Hack to force Flyway to initialize early
Use Hibernate native APIs to defer processing DDL

The EntityManagerFactory will happily process the DDL on startup, but
that happens too early (because of LoadtimeWeaverAware processing). We
can defer it to a more civilised stage, e.g. ContextRefreshedEvent by
using the Hibernate native APIs directly.

It makes the JpaProperties slightly more complex because they need
to distinguish between the early init and late processing versions
of the Hibernate properties.

Not ready for prime time yet because there is no way to deal with
multiple EntityManagers.

Fixes gh-894
Add callback for modifying or inspecting LocalContainerEntityManagerFactoryBean

A callback is added in autoconfig, so that if users inject the EntityManagerFactoryBuilder
into their app and use it to create multiple EntityManagerFactories, they all get the
same deferred DDL behaviour. The deferred DDL can also be disabled by setting
spring.jpa.hibernate.deferDdl=true.

Fixes gh-894