Obtain time from node.

[Jelmerdw]

Description

If I use the moveit_servo servo_node in combination with Gazebo simulation, the moveit_ros.current_state_monitor complains that it doesn't receive robot state with recent timestamp. I launch all nodes with the 'use_sim_time' set to True, however the current_state_monitor still seems to use wall time. A similar problem was explained in this issue, although not for moveit_servo:

#2906

My solution is to change the code from rclcpp::Clock(RCL_ROS_TIME).now() to node_->now(), such that the time is obtained from the node, which is launched with 'use_sim_time' set to True, so that it gives the sim time. This fixes my problem for the moveit_servo node, but will probably not fix the problem in the issue I mentioned.

This is my first contribution. I would like to hear feedback about this suggested change!

Checklist

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[sea-bass]

This looks good to me -- thank you for this fix!