pgsql: Fix logical decoding error when system table w/ toast isrepeate

Fix logical decoding error when system table w/ toast is repeatedly rewritten.

Repeatedly rewriting a mapped catalog table with VACUUM FULL or
CLUSTER could cause logical decoding to fail with:
ERROR, "could not map filenode \"%s\" to relation OID"

To trigger the problem the rewritten catalog had to have live tuples
with toasted columns.

The problem was triggered as during catalog table rewrites the
heap_insert() check that prevents logical decoding information to be
emitted for system catalogs, failed to treat the new heap's toast table
as a system catalog (because the new heap is not recognized as a
catalog table via RelationIsLogicallyLogged()). The relmapper, in
contrast to the normal catalog contents, does not contain historical
information. After a single rewrite of a mapped table the new relation
is known to the relmapper, but if the table is rewritten twice before
logical decoding occurs, the relfilenode cannot be mapped to a
relation anymore.  Which then leads us to error out.   This only
happens for toast tables, because the main table contents aren't
re-inserted with heap_insert().

The fix is simple, add a new heap_insert() flag that prevents logical
decoding information from being emitted, and accept during decoding
that there might not be tuple data for toast tables.

Unfortunately that does not fix pre-existing logical decoding
errors. Doing so would require not throwing an error when a filenode
cannot be mapped to a relation during decoding, and that seems too
likely to hide bugs.  If it's crucial to fix decoding for an existing
slot, temporarily changing the ERROR in ReorderBufferCommit() to a
WARNING appears to be the best fix.

Author: Andres Freund
Discussion: https://postgr.es/m/20180914021046.oi7dm4ra3ot2g2kt@alap3.anarazel.de
Backpatch: 9.4-, where logical decoding was introduced

Branch
------
REL9_6_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/a88482dd24aafeca555cf80aa58cf9ae39f25f9d

Modified Files
--------------
contrib/test_decoding/expected/rewrite.out      | 75 +++++++++++++++++++++++++
contrib/test_decoding/sql/rewrite.sql           | 42 +++++++++++++-
src/backend/access/heap/heapam.c                | 11 +++-
src/backend/access/heap/rewriteheap.c           | 19 ++++++-
src/backend/replication/logical/reorderbuffer.c | 25 +++++++--
src/include/access/heapam.h                     |  1 +
6 files changed, 163 insertions(+), 10 deletions(-)