Summary:
Backfill does not correctly restore its state after TServer restart.
As result it could insert the same vector twice.
Jira: DB-16994

Test Plan: PgVectorIndexTest.ManyRowsWithBackfillAndRestart/*

Reviewers: arybochkin, #db-approvers

Reviewed By: arybochkin

Subscribers: svc_phabricator, ybase, yql

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D44449