To allow `rad sync` to break early from the call to `announce`,
control flow is introduced for the callback.
In `rad sync`, the number of synced nodes is checked against the
`replicas` -- if specified -- and sends a `Break`.
Signed-off-by: Fintan Halpenny <fintan.halpenny@gmail.com>
X-Clacks-Overhead: GNU Terry Pratchett