Fix field names and behavior in SlotRecord.

The field names used in SlotRecord got out of sync with the changes made
to the database schema. Nobody noticed, because these fields are not yet
written to or read from the database.

This change also rewrites SlotRecordTest and adds several sanity checks
to SlotRecord, in preparation of the introduction of slot_origin.

Change-Id: Ifdf040174705bf88104b8c922c9d6d1120d48f3a