pgsql: Fix data loss at inplace update after heap_update().

Fix data loss at inplace update after heap_update().

As previously-added tests demonstrated, heap_inplace_update() could
instead update an unrelated tuple of the same catalog.  It could lose
the update.  Losing relhasindex=t was a source of index corruption.
Inplace-updating commands like VACUUM will now wait for heap_update()
commands like GRANT TABLE and GRANT DATABASE.  That isn't ideal, but a
long-running GRANT already hurts VACUUM progress more just by keeping an
XID running.  The VACUUM will behave like a DELETE or UPDATE waiting for
the uncommitted change.

For implementation details, start at the systable_inplace_update_begin()
header comment and README.tuplock.  Back-patch to v12 (all supported
versions).  In back branches, retain a deprecated heap_inplace_update(),
for extensions.

Reported by Smolkin Grigory.  Reviewed by Nitin Motiani, (in earlier
versions) Heikki Linnakangas, and (in earlier versions) Alexander
Lakhin.

Discussion: https://postgr.es/m/CAMp+ueZQz3yDk7qg42hk6-9gxniYbp-=bG2mgqecErqR5gGGOA@mail.gmail.com

Branch
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/7354b680ab64fbde6072f248fe3b2ff909a99d12

Modified Files
--------------
src/backend/access/heap/README.tuplock             |  11 +
src/backend/access/heap/heapam.c                   | 248 +++++++++++++++++++--
src/backend/access/index/genam.c                   | 138 ++++++++++++
src/backend/catalog/index.c                        |  38 +---
src/backend/catalog/toasting.c                     |  31 ++-
src/backend/commands/dbcommands.c                  |  34 +--
src/backend/commands/vacuum.c                      |  32 ++-
src/include/access/genam.h                         |   9 +
src/include/access/heapam.h                        |   7 +
.../isolation/expected/intra-grant-inplace-db.out  |  10 +-
.../isolation/expected/intra-grant-inplace.out     |  16 +-
.../isolation/specs/intra-grant-inplace-db.spec    |   1 -
src/test/isolation/specs/intra-grant-inplace.spec  |   4 +-
13 files changed, 485 insertions(+), 94 deletions(-)