MXS-852: Store the internal ID in the buffer

If the internal ID is stored in the buffer when it is moving inside the
readwritesplit router, the RWBackend can manage the execution of all
commands with a statement ID by replacing the stored ID with the correct
value.

include/maxscale/session_command.hh

@@ -54,10 +54,11 @@ public:
     uint64_t get_position() const;
 
     /**
-     * @brief Creates a copy of the internal buffer
-     * @return A copy of the internal buffer
+     * @brief Creates a deep copy of the internal buffer
+     *
+     * @return A deep copy of the internal buffer or NULL on error
      */
-    mxs::Buffer copy_buffer() const;
+    GWBUF* deep_copy_buffer();
 
     /**
      * @brief Create a new session command