Avoid hot standby cancels from VAC FREEZE

VACUUM FREEZE generated false cancelations of standby queries on an
otherwise idle master. Caused by an off-by-one error on cutoff_xid
which goes back to original commit.

Backpatch to all versions 9.0+

Analysis and report by Marco Nenciarini

Bug fix by Simon Riggs

src/backend/access/heap/heapam.c

@@ -7572,9 +7572,12 @@ heap_xlog_freeze_page(XLogReaderState *record)
 	if (InHotStandby)
 	{
 		RelFileNode rnode;
+		TransactionId latestRemovedXid = cutoff_xid;
+
+		TransactionIdRetreat(latestRemovedXid);
 
 		XLogRecGetBlockTag(record, 0, &rnode, NULL, NULL);
-		ResolveRecoveryConflictWithSnapshot(cutoff_xid, rnode);
+		ResolveRecoveryConflictWithSnapshot(latestRemovedXid, rnode);
 	}
 
 	if (XLogReadBufferForRedo(record, 0, &buffer) == BLK_NEEDS_REDO)