Periodic snapshotting for external backups.

[purplefox]

We should allow for periodic snapshots to be taken of the shards, either written to the filesystem or pushed to S3.

[severb]

https://kubernetes.io/docs/concepts/storage/volume-snapshots/

Volume snapshots provide Kubernetes users with a standardized way to copy a volume's contents at a particular point in time without creating an entirely new volume. This functionality enables, for example, database administrators to backup databases before performing edit or delete modifications.

Maybe we can use this for backups, at least for now.