Allow swapping map in Eagon map device

Rolling maps for memory threads in between each map is bad. (To make it worse, we cannot use search highlight to make it easier). I just refuse to roll Eagon's maps.

It would be nice if we could prepare maps beforehand and *exchange* the given Eagon's map in the device with our own.

That way:

- we would be able to prepare map for memory threads
- but people could still run memory maps as before without needing any maps if they want