code: func newPostgresLocker(db *sql.DB) sync.Locker {
  	key := crc32.ChecksumIEEE([]byte("migrations"))
  	return &postgresLocker{
  		key: key,
  		db:  db,
  	}
  }
docs: // NewPostgresLocker returns a new sync.Locker that obtains locks with
  // pg_advisory_lock.