Clean shutdown on exit

[sharnoff]

See #8 (comment).

There's a few pieces to this (beyond what's in #8). Probably best to split them into separate PRs.

• Cleanup on VM informant exit
  • note: Currently (2023-01-10), NeonVM doesn't support clean shutdown, so this is not immediately applicable
• Cleanup on autoscaler-agent exit (e.g., using pod lifecycle hooks + cancelling top-level context)
• Cleanup on scheduler exit
  • Some investigation is required here. The scheduler framework knows when it's being replaced, even without any pod lifecycle hooks. However, the pod stays alive until the eviction timeout is reached.

[sharnoff]

This was marked as done on 2023-07-12, but not actually closed.