Ensure JPA/Security are not fighting with each other on startup

This is *really* nasty (and led me to discover a related bug
https://jira.spring.io/browse/SPR-11844), but fortunately easy to
hide from users once you have a test case.

The problem is that Spring Security registers a `BeanPostProcessor`
to handle `GlobalAuthenticationConfigurerAdapters`, and Boot
registers a `BeanPostProcessor` to handle injecting the packages
to scan into an `EntityManagerFactory` from `@EntityScan`. The
clash comes because the `EntityScanBeanPostProcessor` wants to be
postprocessed by the Security postprocessor, but if the Security
configuration depends on JPA it won't be ready in time.

The fix (or workaround) depending on how you look at it is to
prevent the other bean post processors from taking an interest in
`EntityScanBeanPostProcessor` at all (mark it as synthetic).

Fixes gh-1008