pgsql: Backport addition of rs_old_rel to rewriteheap's state.

Backport addition of rs_old_rel to rewriteheap's state.

Originally part of b89e151054a05f0f6d356ca52e3b725dd0505e53, the
introduction of logical decoding, this is required to backport a
commit introducing error checks defending against recent bugs.

It's possible that extensions calls begin_heap_rewrite(), but it seems
highly unlikely. But if so, they'd break.

Author: Andres Freund
Discussion: https://postgr.es/m/20171215010029.3dxx56vjlymudvwo@alap3.anarazel.de

Branch
------
REL9_3_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/152a56905658b7d9f8927f41f000abac718e1a39

Modified Files
--------------
src/backend/access/heap/rewriteheap.c | 4 +++-
src/backend/commands/cluster.c        | 2 +-
src/include/access/rewriteheap.h      | 2 +-
3 files changed, 5 insertions(+), 3 deletions(-)