Add warming code to the control plane

[philwinder]

Ideally the control plane should be responsible for warming. This means that runners can connect quickly, but marked as warming. This way you don't get into a state where runners are running but not communicating with the control plane because they are too busy pulling shit.

That way the control plane really knows who is ready for scheduling.

Could also assign slots for common models. So they're warm and ready to go as soon as they've stopped warming.