This patch changes separates the business logic of fetching from the process of
fetching itself. It does this through a sans-IO approach, where a `Fetcher`
provides the necessary state to help drive a fetch forward, without performing
any of the IO itself.
What the `Fetcher` cares about is:
- What nodes are going to be attempted to be fetched from
- In what order should they be attempted
- When should the fetching process be considered finished
The `Fetcher` is then used in `radicle-cli` to drive forward the `sync::fetch`
function, allowing it to only care about the IO.
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