celery: use explicit task names - avoid automatic naming with "kallithea.lib.celerylib." prefix

We wrap async functions in a local f_async wrapper, defined in
kallithea/lib/celerylib/__init__.py . For a function Foo.X, even though we
changed the wrapper's __name__ to X, the tasks would be named
kallithea.lib.celerylib.X , without using the actual module name of X for
namespacing.

Drop modifying __name__, and just specify the name explicitly, without trying
to namespace it.