Remove duplicate response protos

[devoxel]

Ross pointed out that our response protos should probably be de-deplicated where possible. I think this would probably be quite helpful, we could write util code that would check any response proto and log it if there was an error.

[CianLR]

Something such as

message RabbleResponse {
  enum ResponseType {
    OK = 0
    ERROR = 1
  }

  ResponseType response_type = 1;
  string err = 2;

  FollowsResponse follows_response = 3;
  DatabaseResponse database_response = 4;
  ...
}

The service response messages there can throw away the enum stuff and only include the bits needed. Since they're optional it shouldn't be an issue to have them all combined I think.

[devoxel]

Would it make sense to make a ResponseStatus thats included in other responses? Your way seems like it could get quite big, and includes a lot of info from other sources.