Check for conflicting queries during replay of gistvacuumpage()

013ebc0a7b implements so-called GiST microvacuum.  That is gistgettuple() marks
index tuples as dead when kill_prior_tuple is set.  Later, when new tuple
insertion claims page space, those dead index tuples are physically deleted
from page.  When this deletion is replayed on standby, it might conflict with
read-only queries.  But 013ebc0a7b doesn't handle this.  That may lead to
disappearance of some tuples from read-only snapshots on standby.

This commit implements resolving of conflicts between replay of GiST microvacuum
and standby queries.  On the master we implement new WAL record type
XLOG_GIST_DELETE, which comprises necessary information.  On stable releases
we've to be tricky to keep WAL compatibility.  Information required for conflict
processing is just appended to data of XLOG_GIST_PAGE_UPDATE record.  So,
PostgreSQL version, which doesn't know about conflict processing, will just
ignore that.

Reported-by: Andres Freund
Diagnosed-by: Andres Freund
Discussion: https://postgr.es/m/20181212224524.scafnlyjindmrbe6%40alap3.anarazel.de
Author: Alexander Korotkov
Backpatch-through: 9.6

src/include/access/gist_private.h

@@ -240,6 +240,7 @@ typedef struct GistSplitVector
 typedef struct
 {
 	Relation	r;
+	Relation	heapRel;
 	Size		freespace;		/* free space to be left */
 
 	GISTInsertStack *stack;
@@ -389,7 +390,8 @@ extern void freeGISTstate(GISTSTATE *giststate);
 extern void gistdoinsert(Relation r,
 			 IndexTuple itup,
 			 Size freespace,
-			 GISTSTATE *GISTstate);
+			 GISTSTATE *GISTstate,
+			 Relation heapRel);
 
 /* A List of these is returned from gistplacetopage() in *splitinfo */
 typedef struct
@@ -404,7 +406,8 @@ extern bool gistplacetopage(Relation rel, Size freespace, GISTSTATE *giststate,
 				OffsetNumber oldoffnum, BlockNumber *newblkno,
 				Buffer leftchildbuf,
 				List **splitinfo,
-				bool markleftchild);
+				bool markleftchild,
+				Relation heapRel);
 
 extern SplitedPageLayout *gistSplit(Relation r, Page page, IndexTuple *itup,
 		  int len, GISTSTATE *giststate);
@@ -412,7 +415,7 @@ extern SplitedPageLayout *gistSplit(Relation r, Page page, IndexTuple *itup,
 extern XLogRecPtr gistXLogUpdate(Buffer buffer,
 			   OffsetNumber *todelete, int ntodelete,
 			   IndexTuple *itup, int ntup,
-			   Buffer leftchild);
+			   Buffer leftchild, RelFileNode *hnode);
 
 extern XLogRecPtr gistXLogSplit(bool page_is_leaf,
 			  SplitedPageLayout *dist,