test: cleanup cluster.cas vs instance.cas inconsistencies

Summary

• test: reduce test_log_rollback flakiness

• test: what was this about?

• test: cleanup cluster.cas vs instance.cas inconsistencies

Note that now we don't have any (is this right?) tests that use pico.cas which should be fine considering we plan to get rid of it in the near future.

Also we don't use password for cas operations, because the stored procedure can only be called by the superuser and the access control checks rely on the as_user parameter.

• Cherry-pick to: none
• Docs follow-up: not necessary