Improve transcoding process

[evillemez]

Currently the transcode process transcodes new files for a Resource in the same process, one after the other. So, the process either entirely succeeds, or entirely fails. It would be more efficient to have the app farm out the file transcodes individually, rather than Resource by Resource.

The new config format makes this possible, but there would need to be a reliable cache to know the state of failed/executing/scheduled jobs, in order to know once an entire Resource has completed transcoding.