Revert MERGE patch

This reverts commits d204ef6377,
83454e3c2b and a few more commits thereafter
(complete list at the end) related to MERGE feature.

While the feature was fully functional, with sufficient test coverage and
necessary documentation, it was felt that some parts of the executor and
parse-analyzer can use a different design and it wasn't possible to do that in
the available time. So it was decided to revert the patch for PG11 and retry
again in the future.

Thanks again to all reviewers and bug reporters.

List of commits reverted, in reverse chronological order:

 f1464c5380 Improve parse representation for MERGE
 ddb4158579 MERGE syntax diagram correction
 530e69e59b Allow cpluspluscheck to pass by renaming variable
 01b88b4df5 MERGE minor errata
 3af7b2b0d4 MERGE fix variable warning in non-assert builds
 a5d86181ec MERGE INSERT allows only one VALUES clause
 4b2d44031f MERGE post-commit review
 4923550c20 Tab completion for MERGE
 aa3faa3c7a WITH support in MERGE
 83454e3c2b New files for MERGE
 d204ef6377 MERGE SQL Command following SQL:2016

Author: Pavan Deolasee
Reviewed-by: Michael Paquier

src/include/access/heapam.h

@@ -53,34 +53,23 @@ typedef enum LockTupleMode
  * When heap_update, heap_delete, or heap_lock_tuple fail because the target
  * tuple is already outdated, they fill in this struct to provide information
  * to the caller about what happened.
- *
- * result is the result of HeapTupleSatisfiesUpdate, leading to the failure.
- * It's set to HeapTupleMayBeUpdated when there is no failure.
- *
  * ctid is the target's ctid link: it is the same as the target's TID if the
  * target was deleted, or the location of the replacement tuple if the target
  * was updated.
- *
  * xmax is the outdating transaction's XID.  If the caller wants to visit the
  * replacement tuple, it must check that this matches before believing the
  * replacement is really a match.
- *
  * cmax is the outdating command's CID, but only when the failure code is
  * HeapTupleSelfUpdated (i.e., something in the current transaction outdated
  * the tuple); otherwise cmax is zero.  (We make this restriction because
  * HeapTupleHeaderGetCmax doesn't work for tuples outdated in other
  * transactions.)
- *
- * lockmode is only relevant for callers of heap_update() and is the mode which
- * the caller should use in case it needs to lock the updated tuple.
  */
 typedef struct HeapUpdateFailureData
 {
-	HTSU_Result result;
 	ItemPointerData ctid;
 	TransactionId xmax;
 	CommandId	cmax;
-	LockTupleMode	lockmode;
 } HeapUpdateFailureData;
 
 
@@ -173,7 +162,7 @@ extern void heap_abort_speculative(Relation relation, HeapTuple tuple);
 extern HTSU_Result heap_update(Relation relation, ItemPointer otid,
 			HeapTuple newtup,
 			CommandId cid, Snapshot crosscheck, bool wait,
-			HeapUpdateFailureData *hufd);
+			HeapUpdateFailureData *hufd, LockTupleMode *lockmode);
 extern HTSU_Result heap_lock_tuple(Relation relation, HeapTuple tuple,
 				CommandId cid, LockTupleMode mode, LockWaitPolicy wait_policy,
 				bool follow_update,