Wire up the new `FetcherService`. This reduces the `Service` fetch
methods down to:
- `fetch`
- `fetched`
- `dequeue_fetches`
- `fetch_refs_at`
This simplifies the code by off-loading the logic to the `fetcher`
family of types, rather than handling the intricacies in the `Service`
itself.
The `Service` now just performs the necessary I/O based on the
returned events from the `fetcher`.
This also removes the fetching state from the node sessions. This
follows the single responsibility, and sessions now only care about
their connectivity.
Breaking changes:
- The `Connected` state of a peer no longer contains fetching
information, which was being returned as part of the JSON payload
result.
- The `rad debug` information for ongoing fetches contained the number
listeners awaiting for results, this was removed.
Lorenz Leutgeb
Fintan Halpenny