upgrader: record checksum for integrity checks

For safety, record the checksum of a transient in shard persistence every time the shard goes into `Available` or `Serving` state.

Expose the checksum to the `mount.Upgrader`, so that when the mount needs to be redownloaded, it can perform an integritiy check.

We gotta be careful about recording an invalid checksum though. This can be a footgun if we get it wrong (e.g. recording a bad checksum as good will make future correct downloads error out).