Utilities and cleanups for socket. (#9576)

- Use c++-17 nodiscard and nested ns.
- Add bind method to socket.
- Remove rabit parameters.

tests/cpp/common/test_json.cc

@@ -4,6 +4,7 @@
 #include <gtest/gtest.h>
 
 #include <fstream>
+#include <iterator>  // for back_inserter
 #include <map>
 
 #include "../../../src/common/charconv.h"
@@ -691,4 +692,16 @@ TEST(Json, TypeCheck) {
     ASSERT_NE(err.find("foo"), std::string::npos);
   }
 }
+
+TEST(Json, Dump) {
+  auto str = GetModelStr();
+  auto jobj = Json::Load(str);
+  std::string result_s = Json::Dump(jobj);
+
+  std::vector<char> result_v = Json::Dump<std::vector<char>>(jobj);
+  ASSERT_EQ(result_s.size(), result_v.size());
+  for (std::size_t i = 0; i < result_s.size(); ++i) {
+    ASSERT_EQ(result_s[i], result_v[i]);
+  }
+}
 }  // namespace xgboost