Since we now have three announcement message types, we need at least
three timestamps per peer. But even that is not enough, since we don't
want to conflate ref announcements for different projects with each
other.
The solution is to also keep a map of timestamps per peer, for each
tracked project. That way we are able to know when a ref announcement is
interesting to us or not.
Signed-off-by: Alexis Sellier <alexis@radicle.xyz>