Do main process in callback function

[tongtybj]

I can not understand the motivation to use a independent while scope to do the main function: https://github.com/kam3k/laser_line_extraction/blob/master/src/line_extraction_node.cpp#L24-L29

Actually, for the consistency of the sensor flow, it is better to do the main process in sensor callback function, and use the same timestamp with the sensor' one.