Added support for unique indexes.

So, now your initial idea about resetting during the first phase appears to be ready.

Next step - use single-scan and auxiliary index for concurrent index build.

Also, I have updated the stress tests accordingly to [0].

[0]: https://www.postgresql.org/message-id/flat/CANtu0ojmVd27fEhfpST7RG2KZvwkX%3DdMyKUqg0KM87FkOSdz8Q%40mail.gmail.com

Best regards,

Mikhail.