pk: Make hoard.chunks.last_access NOT NULL
pk: Make hoard.chunks.last_access NOT NULL

- Add NOT NULL constraint to last_access timestamp, and set the column
to 0 if unknown

- Bump HOARD_INDEX_VERSION

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr