Use python-retask to distribute cache refreshing to a worker proc.

This is a first revision/proof-of-concept and needs a lot of cleanup.
It works though!  Async refills of the cache are marked in a redis queue.
A worker proc picks tasks out of that queue, computes the value, places
the new value in the memcached cache, and releases the distributed
dogpile mutex.
Tweak to get koji connector working.
Tweak to get yum connector working.
Specfile updated with new deps.
Merge pull request #2 from fedora-infra/feature/long-running-queue

Feature/long running queue