Some updates to the backup format:

  - The name of the descriptor file is now based on the timestamp in UTC,
    not local time.  This should give better behavior in the case of
    frequent snapshots and daylight saving time changes.
  - Update the backup format to v0.11 (other changes may yet come before
    release); with this also include a name change from "LBS" to "Cumulus"
    in the format name.