Handle "rendering_in_process" state

If the service is already handling a request for the cachekey
return for now. That page request will not get any ads.
Otherwise, either return the value from the cache or issue the
call to the service.

Pass on the 'mcache' variable to the service.
Merge pull request #4513 from Wikia/MON-151-concurrent-requests

[MON-151] Handle "rendering_in_process" state