The `Announcer` logic was subtly different to the fetching logic, where the
preferred seeds need to be announced to *and* the replication factor had to be
reached.
This patch relaxes that constraint, and allows it to hit either targets before
exiting.
This also surfaced an issue with the with the `Progress` calculation, where
calculating the `unsynced` used the `synced` value to subtract. However, this
was not required because `to_sync` is mutated, so the value is simply
`to_sync.len()`. A test case was added to ensure this is correct by checking
that invariant that the total between synced and unsynced nodes always matches
the total of synced and unsynced progress.
8a6e55502c