Update last publish time adding period

[efernandez]

This change is important so we don't ignore the offset between the expected and actual publish time. Otherwise, frequencies that aren't a multiple of the control frequency can never be achieved.

Indeed, this changes makes things match the implementation in the joint_state_controller, that is working as intended:
https://github.com/ros-controls/ros_controllers/blob/12a70baee38bf198ad434ad704b353621b23c61a/joint_state_controller/src/joint_state_controller.cpp#L86