pgsql: Fix replica identity check for MERGE.

Fix replica identity check for MERGE.

When executing a MERGE, check that the target relation supports all
actions mentioned in the MERGE command. Specifically, check that it
has a REPLICA IDENTITY if it publishes updates or deletes and the
MERGE command contains update or delete actions. Failing to do this
can silently break replication.

Author: Zhijie Hou <houzj(dot)fnst(at)fujitsu(dot)com>
Reviewed-by: Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>
Reviewed-by: Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com>
Tested-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Discussion: https://postgr.es/m/OS3PR01MB57180C87E43A679A730482DF94B62@OS3PR01MB5718.jpnprd01.prod.outlook.com
Backpatch-through: 15

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/5481cc332b0fc8ca242966813a6675a514c6917b

Modified Files
--------------
src/backend/executor/execMain.c | 27 ++++++++++++++++++++++++---
src/backend/executor/execPartition.c | 6 ++++--
src/backend/executor/nodeModifyTable.c | 6 +++++-
src/include/executor/executor.h | 3 ++-
src/test/regress/expected/publication.out | 28 ++++++++++++++++++++++++++++
src/test/regress/sql/publication.sql | 31 +++++++++++++++++++++++++++++++
6 files changed, 94 insertions(+), 7 deletions(-)