improve path crop function, add global replan rate as a param, update…

[XuRobotics]

1. improve path crop function (that generates local goal) to handle global start outside local map case which happens often in cluttered environments or when computation burden is too high,
2. add global replan rate as a param so that the user can slow down global replan rate if needed,
3. update mapper config to avoid issues with running example on computers with very limited resource