Sorting speed-up: Cache the bool value of Transaction's is_closing property.

This value is queried on each comparison of split or txn sort function,
which means it is called quite a lot. Avoiding the KVP lookup of this
property gains a lot in terms of CPU cycles.

libgnucash/engine/TransactionP.h

@@ -119,6 +119,11 @@ struct transaction_s
      */
     char * readonly_reason;
     gboolean reason_cache_valid;
+
+    /* Cached bool value to indicate whether this is a closing txn. This is
+     * cached from the KVP value because it is queried a lot. Tri-state value: -1
+     * = uninitialized; 0 = FALSE, 1 = TRUE. */
+    gint isClosingTxn_cached;
 };
 
 struct _TransactionClass