I'm thinking of removing that 2nd type parameter in Store, Service, Sink, ...
This is the actual type used by the platform, but the Producer API does not care at that point because those types have been erased by the scala compiler and replaced by Object.
The downside of that parameter is the summingbird job code must use "?" to say that is is not using it. The upside is that otherwise the Wrapper class would just contain Object when possibly we may want to access the actual type to interact with it.