Move large query processing inside RWBackend

The knowledge of which function to call can be internal to RWBackend. This
make the use of the class easier as one can simply write to the backend.

include/maxscale/protocol/rwbackend.hh

@@ -115,21 +115,6 @@ public:
      */
     bool write(GWBUF* buffer, response_type type = EXPECT_RESPONSE);
 
-    /**
-     * Continue a previously started write
-     *
-     * This should only be used when RWBackend::write has been called to start
-     * a new query.
-     *
-     * @param buffer Buffer to write
-     *
-     * @return True if writing was successful
-     */
-    bool continue_write(GWBUF* buffer)
-    {
-        return mxs::Backend::write(buffer, Backend::NO_RESPONSE);
-    }
-
     void close(close_type type = CLOSE_NORMAL);
 
     // For COM_STMT_FETCH processing
@@ -181,6 +166,7 @@ private:
     bool             m_local_infile_requested;  /**< Whether a LOCAL INFILE was requested */
     ResponseStat     m_response_stat;
     uint64_t         m_num_coldefs = 0;
+    bool             m_large_query = false;
 
     inline bool is_opening_cursor() const
     {