pg_autovacuum looses track of any table that's ever been truncated
(possibly other situations too).   When i truncate a table it gets a
new relfilenode in pg_class.  This is a problem because pg_autovacuum
assumes pg_class.relfilenode will join to pg_stats_all_tables.relid.
pg_stats_all_tables.relid is actallly the oid from pg_class, not the
relfilenode.   These two values start out equal so pg_autovacuum works
initially, but it fails later on because of this incorrect assumption.

This patch fixes that problem.  Applied to HEAD and 7.4.X.

Brian Hirt