GJ-4: Remove the need for the Result interface

This brings a lot of code duplication between rpc-dispatch and rest-dispatch. Previously we had rest-dispatch inheriting common code from rpc-dispatch. This is no longer possible because rest-dispatch removed the Result interface from its requirements. As far as I know, the opposite should be possible (rpc-dispatch could inherit some code from rest-dispatch and further enforce the Result interface).
I'd like to go further though and have a gwtp-dispatch-client-common project for common code and remove any remaining dependencies between rpc and rest-dispatch. I think the refactor should be part of another story.

Again, I did only the necessary changes to have the carstore compiling. My next PR will fix any remaining issues.