MXS-2415: Fix client callbacks in arvorouter

The DCB callbacks shouldn't be used to send more events as they cause the
callback to be called recursively. The recursive calls caused rows to be
sent before the schemas for the rows were sent. Queuing the events via the
worker mechanism prevents this.

server/modules/routing/avrorouter/avrorouter.hh

@@ -170,11 +170,6 @@ public:
      */
     int routeQuery(GWBUF* buffer);
 
-    /**
-     * Handler for the EPOLLOUT event
-     */
-    void client_callback();
-
 private:
     AvroSession(Avro* instance, MXS_SESSION* session);
 
@@ -187,6 +182,8 @@ private:
     bool seek_to_gtid();
     bool stream_data();
     void rotate_avro_file(std::string fullname);
+    void client_callback();
+    void queue_client_callback();
 };
 
 void read_table_info(uint8_t* ptr,