Fixed timestamp problem, on-demand marker publishing, etc.

[salihmarangoz]

Hello there,

I found several issues in your code and fixed them:

• Output timestamps must be equal to the input laser scan timestamp. Because some computers may be slow and some others may be faster. This will be important for some nodes (e.g. SLAM based on line segments).
• RViz Marker publishing topic is now working on-demand. Adding parameters for visualization topics is not needed. When there are no subscribers, no CPU resources will be used for these markers.
• "frequency" parameter was a bit weird. If the laser scan arrives at 10hz but frequency is 25, line segments are being published at 25hz. This is not a good thing for nodes with heavy algorithms using line segments as inputs. I have modified the code and now the "frequency" parameter works as the maximum rate of processing inputs.