Declare rclcpp callbacks before the rcl entities (ros2#2024)

This is to ensure callbacks are destroyed last
on entities destruction, avoiding the gap in time
in which rmw entities hold a reference to a
destroyed function.

Signed-off-by: Mauro Passerino <mpasserino@irobot.com>

rclcpp/include/rclcpp/client.hpp

@@ -406,9 +406,6 @@ class ClientBase
   bool use_intra_process_{false};
   IntraProcessManagerWeakPtr weak_ipm_;
   uint64_t intra_process_client_id_;
-
-  std::recursive_mutex callback_mutex_;
-  std::function<void(size_t)> on_new_response_callback_{nullptr};
 };
 
 template<typename ServiceT>