Refactor of Stage to Core communication

This is a preparation for allowing different implementations of the Stage to Core transport (i.e. `RemotePipeline` and the `com.findwise.hydra.net` package).

I'm not sure if it is best to merge the extraction of the interface and the reformatting right away, or if this should be a work in progress PR.

My idea is that with the `RemotePipeline` interface, alternative implementations of the transport are possible from the stage side. It is more complicated on the Core side, but perhaps other implementations don't have to conform with how the REST/HTTP impl works. It's a bit hard to extract interfaces or do other abstractions with the `RESTServer` and the rest of the `com.findwise.hydra.net` classes.