Delegated Delivery: Episode file metadata stuck in null state

[svevang]

In the case where the podcast container is synced from remote state, and the episode and container can have needs_delivery? == false. But since the podcast container sync routine resets the podcast containers for expired source urls with each pass, we can end up:

1. Not needing to refresh the metadata
2. Probing the metadata with a nil url

In the case of 1 and 2, the routines are dependent on each other. And it's assumed that the metadata is present when we HEAD the CDN url in 2.

We should have matching criteria in these two methods. That is: if the episode needs_delivery? == true we should reset the source metadata and likewise we should only probe source file metadata if we're intending to upload the media later in the publishing routine (e.g. we filter out the episodes needing delivery when creating the delivery file models)