ClassConfigurator: use constructors rather than Classes in map

Investigate whether use of Constructor as values in the maps rather than Class<?> improves performance. Class.newInstance() does cache the constructor, but there's a security check that's executed once, so using the constructor may improve perf.
Also, if a class doesn't have an empty constructor, using Class.newInstance() would throw an exception, whereas using constructors would detect this at load time.