[ODE] Memory management

[Megan Fox]

I might be mis-remembering, but wasn't this a change that was added to
lend some additional stability to the sim?

(what I mean is, isn't this something that should be easy to remove,
if needed, and would it benefit from having a switch command to turn
it off for those that want repeatable sims at the cost of less
stability?)

-Megan Fox

> 2) ODE is repeatable if you use dWorldStep(), and fixed physics step sizes,
> assuming the inputs are the same. For example, if you leak bodies in
> shutdown, that means the matrices are different in the next iteration,
> because more bodies are involved. Also, if your near-callbacks use random
> numbers, they need to be made repeatable, too. Last, if you use
> dworldQuickStep(), then there's a random re-ordering of the constraints,
> which you have to re-seed in a repeatable manner.