Create and use MachineId type

[tmpfs]

This will be the equivalent Uuid to the VaultId of the device vault.

This identifier should then be stored in the manifest for backup archives and persisted to disc. Upon restoring a backup archive we can:

1. Read machine ID from the manifest
2. Read machine ID from the device
3. If they match restore the device vault
4. if they don't match, ignore restoring the device vault (a new one will be lazily created later)

The reason for this is that device public keys determine server authorization and it would be a nice detail to allow restoring to an existing device and be able to subsequently immediately sync with remote servers using a device key that has already been authorized on those server(s).