Migrate task logs from redis and database to logs files
Having logs stored in redis then in database took too much memory in redis and storage in database. Using files is far simplier, but it require to have a shared file system (nfs) when the worker and the cubiweb instance (the reader) are not in the same server. Use the new cw_celerytask_helpers filelogger instead of redislogger. Logs are stored in celerytask-log-dir directory in gzip with a predictible filename based on task_id (which is unique). Drop task_logs attribute from CeleryTask and update tests accordingly. celery-monitor don't copy anymore from redis to database when the task is ended.