Ensures that preferred seeds that are not synced are made part of the `to_sync`
set, and the Announcer is constructed without an error.
As well as this, ensure that the `synced` and `to_sync` sets do not overlap by
constructing the `to_sync` and removing any `synced` elements from it.
Making a note of that `Announcer::can_continue` might never be able to hit the
`Break` case. It could be defensive programming, so I'm going to leave it in for
the time being.
Adds a test case to show that the Announcer is allowed to mark an "unknown" node
as synced.
There may be a case where gossip reached a node that the current node was not
aware of. This should be allowed and should count towards the final replication
factor.
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.
Lorenz Leutgeb
Fintan Halpenny