Use numbers module
Redis backend chord optimization: Avoid save group at apply and pipeline + O(1) join.

This change is backward incompatible and so is not enabled by default.

To enable this optimization you have to set the `new_join` option
and it must be enabled by all clients and workers part of the chord::

    redis://?new_join=1
Merge branch 'master' of github.com:celery/celery