Core: create transactions by annotation (#698).

A plugin developer can create a transaction just by annotating a resource method. (A resource method is one with a method designator like @GET.) The DM Core takes provisions to "wrap" such an annotated method within a transaction automatically.

Example:
{{{
    import de.deepamehta.core.service.Transactional;

    @Transactional
    public void aResourceMethod() {
        ...
    }
}}}

See #698.