Implementing `Closeable` is interesting. Currently there's `EntityInternal.destroy()` and `AbstractEntity.onManagedStopped()`. I don't think we should call it `close()` as that might well collide with an effector name to "close" the underlying entity in some way.

However, `destroy` is not called properly and not used by entity implementations. We should change that at some point (but not in this PR).