Stop duplicate rows in cs_indexer_state

The cs_indexer_state table (which represents InternalChangesetIndexerState) does not have a uniqueness constraint on its primary key.

Customers have reported duplicate rows in the table, which causes issues with deleting repositories.

1. Write a liquibase patch to delete duplicate rows in cs_indexer_state
   • Must be run before adding the uniqueness constraint!
   • Ideally the liquibase patch should first find all duplicate rows, then delete them and reconstruct 1 row for each set of duplicates.
2. Write liquibase patch to add a constraint for unique primary key
3. Track down the cause of duplicate rows