Range messages buffer should have a maximum size

[corot]

In my case, costmap update rate cannot cope with all the incoming readings, so buffer starts growing out of control. and so too old messages are processed after a while.

What I have done is to create a buffer per incoming topic and limit its size with a "range_msgs_buffer_size" parameter. I can PR the code, if you think it's a good idea.

Btw, I can make it a bit faster using C++11 unsorted_map instead of map. Any problem with adding C++11 requirement to range_sensor_layer?

[DLu]

Really late to the game, but please put in a PR for the max buffer size. Thanks!