add user data in connection.

examples/server/main.cpp

@@ -7,6 +7,13 @@ using namespace rpc_service;
 
 struct dummy{
 	int add(rpc_conn conn, int a, int b) {
+		auto shared_conn = conn.lock();
+		if (shared_conn) {
+			shared_conn->set_user_data(std::string("aa"));
+			auto s = conn.lock()->get_user_data<std::string>();
+			std::cout << s << '\n'; //aa
+		}
+
 		return a + b;
 	}
 };