SET DISTRIBUTED BY and EXTEND TABLE subcommands worked differently from
all other ALTER TABLE subcommands in who's responsible for closing the
relcache reference. In all other subcommands, ATRewriteCatalogs opens and
closes the 'rel', but for these two commands, the ATExec*() function
closed it. I don't see any good reason for that. There were very old
comments about forcing the relcache entry to be forgotten, but that
explanation doesn't make sense to me, and everything seems to work without
the early closing. Maybe it was needed a long time ago, but the code has
changed a lot since it was written. Simplify, by closing the relation in
ATRewriteCatalogs(), like with all other ALTER TABLE subcommands.
Reviewed-by: NAsim R P <pasim@vmware.com>