FYI: contributed JpaUserDetailsService & JpaUserDetailsManager
Since I figured someone might want to use JPA as their implementation of UserDetailsService and/or UserDetailsManager, I contributed two classes, JpaUserDetailsService and JpaUserDetailsManager, that do the boilerplate work of the implementation, leaving the meat to subclasses.
The source files are attached to the issue
Maybe they'll get included in a contrib module some time. In the meantime, you can use them for yourselves.