Write mapper as a ROS service

[XuRobotics]

Currently, the mapper is a node that keeps publishing global and local maps regardless of whether any node is using them. This results in unnecessary message publication and computation (e.g. local map is published at 10Hz but the local planner only uses it at 2Hz).

The most efficient way is to write mapper as a ROS service. Whenever the planner needs the map, it will have the mapper-client send a request for this service, and obtain the map at that timestamp.