Convert gist to compute page level xid horizon on primary.

Due to parallel development, gist added the missing conflict
information in c952eae52a, while 558a9165e0 moved that computation
to the primary for the index types that already had it.  Thus adapt
gist to also compute on the primary, using
index_compute_xid_horizon_for_tuples() instead of its own copy of the
logic.

This also adds pg_waldump support for XLOG_GIST_DELETE records, which
previously was not properly present.

Bumps WAL version.

Author: Andres Freund
Discussion: https://postgr.es/m/20190406050243.bszosdg4buvabfrt@alap3.anarazel.de

src/include/access/gist_private.h

@@ -431,7 +431,7 @@ extern XLogRecPtr gistXLogUpdate(Buffer buffer,
 			   Buffer leftchild);
 
 extern XLogRecPtr gistXLogDelete(Buffer buffer, OffsetNumber *todelete,
-			   int ntodelete, RelFileNode hnode);
+			   int ntodelete, TransactionId latestRemovedXid);
 
 extern XLogRecPtr gistXLogSplit(bool page_is_leaf,
 			  SplitedPageLayout *dist,

src/include/access/gistxlog.h

@@ -47,8 +47,7 @@ typedef struct gistxlogPageUpdate
  */
 typedef struct gistxlogDelete
 {
-	RelFileNode hnode;			/* RelFileNode of the heap the index currently
-								 * points at */
+	TransactionId latestRemovedXid;
 	uint16		ntodelete;		/* number of deleted offsets */
 
 	/*

src/include/access/xlog_internal.h

@@ -31,7 +31,7 @@
 /*
  * Each page of XLOG file has a header like this:
  */
-#define XLOG_PAGE_MAGIC 0xD100	/* can be used as WAL version indicator */
+#define XLOG_PAGE_MAGIC 0xD101	/* can be used as WAL version indicator */
 
 typedef struct XLogPageHeaderData
 {