Bug 18887: Fix DB structure issues

Bug 18887: Fix DB structure issues

- Make atomic update idempotent
- Correct COLLAT (atomic update)
- Add index on rule_name (kohastructure.sql)
- Add rule_name to the unique constraint (kohastructure.sql)
- Remove max_holds columns (kohastructure.sql)

Signed-off-by: Lisette Scheer <lisetteslatah@gmail.com>
Signed-off-by: Jesse Maseto <jesse@bywatersolution.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>