celery.contrib.conflator: add a Conflator task subclass

Sometimes life gives you a periodic task which can get backed up
and you only really want one of them to execute and the others are
effectively noops (for instance because they'd poll a remote system
for updates which had just been retrieved). This task type will
not execute any tasks which were scheduled before the most recent
execution occured.

With a memory cache backend this will only apply to a single worker
however if memcached is configured then any pool of workers with
access to the same memcache will be able to share this information.