Fixes#1539
If an actor is `CRASHED`, it can't be resumed/paused/suspended anymore,
it can only be deleted.
An actor deletion will clean up everything under the prefix of
`Actor.external_snapshot.snapshot_uri` and
`Actor.in_progress_snapshot_name` (the latter will be empty if the
underlying worker was deleted).
Before, we risked leaking a snapshot after a worker deletion because we
were clearing the `in_progress_snapshot_name` field. We don't need to
clear it up because the CRASHED state is terminal.
- [x] Tests pass
- [x] Appropriate changes to documentation are included in the PR
Luiz Oliveira
cfaf72a17b