pk: expand_slot_cache() optimizations
pk: expand_slot_cache() optimizations

- No longer count the records in hoard.chunks in order to determine
whether the table is empty; instead, use query_column_types() to
determine whether max(offset) is returning NULL.  This substantially
improves the performance of "isr hoard".

- When creating new slots, set the allocated flag while inserting the
slot records rather than in a separate query

- No longer clear tag, length, and crypto fields when setting the
allocated flag, since they should already be clear

- Add early return to remove indent level

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