Tech doctors/be faster on cache misses 57378292

https://github.com/mezis/yarp/issues/1 :

> When a request is made for something that's not in the cache, we currently synchronously fetch from upstream, and return the results.
> 
> It would be faster, in this case, to redirect to upstream asynchronously (through DJ for instance) prime the cache.
> This is faster for the client on 1st request ; the drawback (which is not our concern) is 2 identical requests made to upstream.

Implementation:
When user hits yarp and there is no entry in cache, then s/he is redirected to rubygems, while task to fetch resource from rubygems is enqueued. There are 4 threads which are listening to the queue for resources to fetch.