Do avro conversion in main thread

The avrorouter no longer uses the housekeeper for the conversion
task. This prevents the deadlock which could occur when clients were
notified at the same time that the binlogrouter was adding a master
reconnection task.

server/modules/routing/avrorouter/avrorouter.hh

@@ -229,6 +229,7 @@ struct Avro
                                           * a flush of all tables */
     uint64_t                 block_size; /**< Avro datablock size */
     enum mxs_avro_codec_type codec; /**< Avro codec type, defaults to `null` */
+    uint32_t                 task_handle; /**< Delayed task handle */
 };
 
 struct AvroSession