(CDAP-864) Fix classloading in unit-test mode

Make class loading of byte code generated classes aware of classes that needs to be preserved with the original ClassLoader so that even user classes presented in both system classpath and program classloader, it can be loaded correctly.
So Handler method arguments can only be defined in the order `HttpServiceRequest request, HttpServiceResponder responder, ..`? Doesn't it make sense to allow any order?