Retrieval deals shouldn't be queued concurrently when CID/miner match

[dineshshenoy]

When a transfer is interrupted or it's not clear that a deal is in-progress, it's common for a human or code to retry. If there is an existing retrieval deal with the same CID/client-id on the miner that is already in the queue, we should just return that deal-ID rather than creating a new deal.

There is an edge case that the price of the retrieval deal could get cheaper, but the request is that price is ignored. The previous deal can be canceled if they want to resubmit the request and go to the end of the queue..