Teach contrib/amcheck to check the unique constraint violation

Teach contrib/amcheck to check the unique constraint violation

Add the 'checkunique' argument to bt_index_check() and bt_index_parent_check().
When the flag is specified the procedures will check the unique constraint
violation for unique indexes.  Only one heap entry for all equal keys in
the index should be visible (including posting list entries).  Report an error
otherwise.

pg_amcheck called with the --checkunique option will do the same check for all
the indexes it checks.

Author: Anastasia Lubennikova <[email protected]>
Author: Pavel Borisov <[email protected]>
Author: Maxim Orlov <[email protected]>
Reviewed-by: Mark Dilger <[email protected]>
Reviewed-by: Zhihong Yu <[email protected]>
Reviewed-by: Peter Geoghegan <[email protected]>
Reviewed-by: Aleksander Alekseev <[email protected]>
Discussion: https://postgr.es/m/CALT9ZEHRn5xAM5boga0qnrCmPV52bScEK2QnQ1HmUZDD301JEg%40mail.gmail.com