Reference count async queue transactions

so we can put them of task message queues

asyncqueue.h

@@ -75,6 +75,16 @@ pAsyncTxn AsyncUnitPrepareTxn(pAsyncUnit unit,
                               AsyncTxnHandler responseHandler,
                               void *context, int rsp_len);
 
+/** \brief clone a pointer to the transaction - must use AsyncUnitFreeTxn
+ * \param txn the transaction to clone
+ */
+pAsyncTxn AsyncUnitHoldTxn(pAsyncTxn txn);
+
+/** \brief free a cloned transaction
+ * \param txn the transaction to free (returned by AsyncUnitHoldTxn)
+ */
+void AsyncUnitFreeTxn(pAsyncTxn txn);
+
 /** \brief prepare and queue a transaction
  *
  * \param unit AsyncUnit