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