MXS-1549: Implement optimistic transaction execution

When the `optimistic_trx` mode is enabled, all transactions are started on
a slave server. If the client executes a query inside the transaction that
is not of a read-only nature, the transaction is rolled back and replayed
on the master.

include/maxscale/queryclassifier.hh

@@ -218,6 +218,16 @@ public:
         return m_trx_is_read_only;
     }
 
+    /**
+     * Check if current transaction is still a read-only transaction
+     *
+     * @return True if no statements have been executed that modify data
+     */
+    bool is_trx_starting() const
+    {
+        return qc_query_is_type(m_route_info.type_mask(), QUERY_TYPE_BEGIN_TRX);
+    }
+
     /**
      * @brief Store and process a prepared statement
      *