Building store and object fixes

Auto-cleanup building objects (those that get flattened) after 5
minutes. Prevents blocking the building store with dead objects.
Added M32 to US Launchers crate.
Removed unpopular building materials from town spawn. Must use building
store instead. Towns now more likely to spawn more useful parts (like
walls and ramps)
Reduced object interaction delay from 0.3 to 0.2