upgrader: record checksum for integrity checks

[raulk]

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).

[aarshkshah1992]

@raulk This too is something that can go in post-M1 and can be removed from the 0.1 Milestone.

[raulk]

There's plenty of time to get this in by M1, and it's net positive for stability.