Fix memory leak in xaccTransGetReadOnly

In addition implement a cache for this value as suggested in the comments
as this function is called on every transaction commit.

libgnucash/engine/TransactionP.h

@@ -110,6 +110,15 @@ struct transaction_s
      * any changes made if/when the edit is abandoned.
      */
     Transaction *orig;
+
+    /* The readonly_reason is a string that indicates why a transaction
+     * is marked as read-only. If NULL, the transaction is read-write.
+     * This value is stored in kvp, but we cache a copy here for
+     * performance reasons. reason_cache_valid indicates whether the
+     * cached value is valid.
+     */
+    char * readonly_reason;
+    gboolean reason_cache_valid;
 };
 
 struct _TransactionClass