Add Model and Configurable interface. (#4945)

* Apply Configurable to objective functions.
* Apply Model to Learner and Regtree, gbm.
* Add Load/SaveConfig to objs.
* Refactor obj tests to use smart pointer.
* Dummy methods for Save/Load Model.

src/common/json.cc

@@ -718,5 +718,13 @@ void Json::Dump(Json json, std::ostream *stream, bool pretty) {
   writer.Save(json);
 }
 
+void Json::Dump(Json json, std::string* str, bool pretty) {
+  GlobalCLocale guard;
+  std::stringstream ss;
+  JsonWriter writer(&ss, pretty);
+  writer.Save(json);
+  *str = ss.str();
+}
+
 Json& Json::operator=(Json const &other) = default;
 }  // namespace xgboost