Pausing costmap converstion when the costmap is stopped

[nirwester]

Currently the converter continuously parses the costmap, even when this is stopped and didn't change from the previous execution. Since the CPU usage of most of the plugins is relevant (often consumes an entire core on our platforms), it would be useful to pause the parsing if the costmap is stopped.

Unfortunately it is currently not easy to check if the costmap is stopped; I made a PR to expose the stopped status of the costmap2D_ros here

ros-planning/navigation#913

Can you think about any alternative that could avoid this unnecessary computation? Maybe comparing the current costmap with the latest used one?