This adds functionality so that on `rad sync` and `rad clone`, we
connect to known seeds if necessary, before attempting to fetch.
In passing, some related changes were made:
* The `rad sync` command's arguments were reworked:
`--replicas <count>` can be used to specify a replica count when
fetching
* The `rad patch` command no longer has a `--fetch` option
* It's now possible to tell the node not to automatically connect to
peers in the background
* `AddressBook` is being used underneath `Seeds` to allow for shuffled
iteration
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