Rollback sequence reset on TRUNCATE rollback patch

The attached patch modifies TRUNCATE ... RESTART IDENTITY so that if the
transaction rolls back the restart of the sequence will also be rolled back.

It follows the general outline discussed at
http://archives.postgresql.org/pgsql-hackers/2008-05/msg00550.php of
assigning a new reffilenode to the sequence.


I will add this to the next commitfest.


Steve