Fix rare missing cancellations in Hot Standby.

The machinery around XLOG_HEAP2_CLEANUP_INFO failed
to correctly pass through the necessary information
on latestRemovedXid, avoiding cancellations in some
infrequent concurrent update/cleanup scenarios.

Backpatchable fix to 9.0

Detailed bug report and fix by Noah Misch,
backpatchable version by me.

src/backend/access/heap/pruneheap.c

@@ -173,7 +173,7 @@ heap_page_prune(Relation relation, Buffer buffer, TransactionId OldestXmin,
 	 * initialize the rest of our working state.
 	 */
 	prstate.new_prune_xid = InvalidTransactionId;
-	prstate.latestRemovedXid = InvalidTransactionId;
+	prstate.latestRemovedXid = *latestRemovedXid;
 	prstate.nredirected = prstate.ndead = prstate.nunused = 0;
 	memset(prstate.marked, 0, sizeof(prstate.marked));