Review prepare_blend operation - priority

[raphaelrpl]

During data cube generation, the task prepare_blend is called in order to organize the period and quality bands. After that, it calls the task blend for each band group. Once blend is dispatched, it re-schedule the data cube generation to the end of queue. It is inconvenient since if there are several tiles to do, it may take too long to finish a data cube. It should be triggered with a high priority task.

Link: Celery Message Priority