Add per spawner (and entity type) caps for entities

[hhyyrylainen]

and make the spawners quite quickly fill it up. This will make it easier to balance the amount of iron and microbes there are in the game swimming around, if the spawn system quickly ramps them up to the max amounts and keeps them hovering over there. For low total population cases the max entities should also be lowered.

[hhyyrylainen]

I'll not try to keep this in the latest milestone anymore as seems like that doesn't help this getting done.

[hhyyrylainen]

With the performance changes etc. this kind of seems unnecessary now. There's still the spawn sector issue open so closing this should be fine.