Reorganize expire code

[joto]

The code was a bit weird, because the decision whether to do full area expire of boundary only is mixed up with the code to do the full area expire. This refactoring fixes that, we decide first what to do and then do it.

What is a bit awkward now is the way we do the box calculation in the decide_expire_mode() function only if needed and then later if it wasn't done already. But if all goes well this will be cleaned up in a future commit when we don't do expire by bounding box any more but by the actual area.