* op-node: Simplify Receipts Fetching API

The previous API would return a receipts fetcher that the caller
was responsible for iterating through and then calling `Result` on.
This moves this inside the `Fetch` function. This simplifies usage
of the API and does not result in performance regression because
the receipts hash check requires all receipts to be fetched before

* Cache receipts fetcher rather than receipts

This modifies the refactor to be equivalent to the previous code.
Caching the receipts fetcher is actually critical for performance
because the fetcher caches intermediate results. The fetcher typically
makes lots of calls to fetch all the receipts so if all work is thrown
out on a single failure it will be very hard for the node to make progress.

* Rename Fetch -> FetchReceipts