in 20190111-unique-delivery-mechanisms we remove a 'redundant' index:

-- The ix_licensepooldeliveries_datasource_identifier_mechanism index
-- is not necessary - an index is automatically created on the same
-- fields to enforce a uniqueness constraint.
drop index if exists ix_licensepooldeliveries_datasource_identifier_mechanism;

But on older circulation managers, this index isn't redundant, because the uniqueness constraint was never created – I added it to the model but didn't add a migration script. Dropping this index is terrible for the performance of the lane query. We need the migration script that should have been added in the first place.