Fix vehicles segfaulting from parts breaking off

Fixes #4497 (definitely), fixes #4549 (I think, it sounds like he had the same problem but I haven't actively confirmed), and obsoletes #4499.

* Vehicles destroyed by damage (as opposed to disassembly) will be properly removed from the map.
* Parts breaking off will refresh the vehicle cache, so the vehicle will no longer look like someone took a car-sized eggbeater to it.

With thanks to @Skyyrunner who dug up enough clues for me to figure out what was going wrong, and also for teaching me that obliterating vehicles with incendiary grenades is ridiculous amounts of fun.