Handle graceful worker termination

[sfunkenhauser]

Part of [#23](#23

The overall plan is:

1. Trap SIGTERM in the ateom, and forward SIGTERM to all application containers.
2. Once all application containers exit, ateom will exit itself.
3. The workerpool syncer will mark workers as 'DRAINING' when a pod deletion is triggered (pod.DeletionTimestamp) to prevent actors from being scheduled. On the pod Deleted event, the worker and actor (if it was assigned) will be cleaned up.

This PR implements 3), there are 2 more PRs coming which will implement 1) and 2) for each ateom (gvisor and uVM).

[sfunkenhauser]

Julian Gutierrez Oschmann (@juli4n) Zoe Zhao (@zoez7) PTAL