Disk cache write producer.

This producer passes through to the next producer in the sequence, as long as the permitted request level reaches beyond the disk cache. Otherwise this is a passive producer.

The final result passed to the consumer put into the disk cache as well as being passed on.

This implementation delegates disk cache requests to BufferedDiskCache.

This producer is currently used only if the media variations experiment is turned on, to enable another producer to sit between cache read and write.