Ability to submit alternative hashes earlier

A common flow is

1. Lookup in indexer

2. Check if any of those items are cached

3. Add item to account, potentially triggering download

The problem is that that flow doesn't usually wait for the download to finish and as a result the data from the indexer that would be most useful as an alternative hash is gone by the time it can be added.

To give a practical example, Newznab indexers usually have an id that would be basically perfect: guid. The only issue is that I realistically only have the guid at the time of the initial create request. It would still be a huge win even if they didn't get applied until the download completed and it got added to cache even though that wouldn't let me easily identify if I'm already downloading it.