Remove depreciated only_one decorator.
Prevent including task_self repr in key name.
Because some tasks are nested in a class, they use a task_self argument instead of the normal self which the celery_once package doesn't recognize to strip out.
I have no idea why this setting name has to be changed. The docs for Celery 4.1.0 (installed) indicate it should be called CELERY_TASK_ALWAYS_EAGER - even with namespace removal TASK_ALWAYS_EAGER doesn't work, but the "old" name of CELERY_ALWAYS_EAGER does.
