node: Use millisecond precision for timestamps

Previously we used seconds, which could result in false positives
when checking for stale announcements, if things happened too quickly.

radicle-node/src/test/gossip.rs

@@ -27,7 +27,7 @@ pub fn messages(count: usize, now: LocalTime, delta: LocalDuration) -> Vec<Messa
         msgs.push(Message::inventory(
             InventoryAnnouncement {
                 inventory: arbitrary::vec(3).try_into().unwrap(),
-                timestamp: time.as_secs(),
+                timestamp: time.as_millis(),
             },
             &signer,
         ))

radicle-node/src/test/peer.rs

@@ -180,7 +180,7 @@ where
     }
 
     pub fn timestamp(&self) -> Timestamp {
-        self.clock().as_secs()
+        self.clock().as_millis()
     }
 
     pub fn git_url(&self, repo: Id, namespace: Option<RemoteId>) -> remote::Url {