Change the overlay priorities

This API sets the overlay priorities, specifically the order of the different overlays' route maps. The priority is a JSON object where the key is the priority, and the value of the key is the overlay ID with that priority.

Example:
{ 1 : 2, 2 : 3, 3 : 1 }
This would order the overlays with the overlay with ID 2 first, overlay with ID 3 second, etc.