fix(iroh-net): reduce noise in swarm discovery due to republish (#2685)

## Description

Since republish occurs more or less every second, for every peer in the
local network, every second we insert it in the node_map (which includes
locking), print a debug line, and for requesters, re-send a value that
was previously sent (since every time a new sender is added we
immediately send known values). This pr makes it so that we do all this
only when new values are found.

## Breaking Changes

- swarm discovery will no longer send consecutive repeated values

## Notes & open questions

the typo is unrelated to the pr but was flagged by ci

## Change checklist

- [x] Self-review.
- [ ] ~~Documentation updates following the [style
guide](https://rust-lang.github.io/rfcs/1574-more-api-documentation-conventions.html#appendix-a-full-conventions-text),
if relevant.~~
- [ ] ~~Tests if relevant.~~
- [x] All breaking changes documented.

iroh-blobs/src/downloader.rs

@@ -411,7 +411,7 @@ impl Downloader {
         }
     }
 
-    /// Declare that certains nodes can be used to download a hash.
+    /// Declare that certain nodes can be used to download a hash.
     ///
     /// Note that this does not start a download, but only provides new nodes to already queued
     /// downloads. Use [`Self::queue`] to queue a download.

iroh-net/src/discovery/local_swarm_discovery.rs

@@ -121,19 +121,28 @@ impl LocalSwarmDiscovery {
                             continue;
                         }
 
-                        if let Some(senders) = senders.get(&discovered_node_id) {
-                            for sender in senders.values() {
-                                let item: DiscoveryItem = (&peer_info).into();
-                                trace!(?item, "sending DiscoveryItem");
-                                sender.send(Ok(item)).await.ok();
+                        let entry = node_addrs.entry(discovered_node_id);
+                        if let std::collections::hash_map::Entry::Occupied(ref entry) = entry {
+                            if entry.get() == &peer_info {
+                                // this is a republish we already know about
+                                continue;
                             }
                         }
+
                         debug!(
                             ?discovered_node_id,
                             ?peer_info,
                             "adding node to LocalSwarmDiscovery address book"
                         );
-                        node_addrs.insert(discovered_node_id, peer_info);
+
+                        if let Some(senders) = senders.get(&discovered_node_id) {
+                            let item: DiscoveryItem = (&peer_info).into();
+                            trace!(?item, senders = senders.len(), "sending DiscoveryItem");
+                            for sender in senders.values() {
+                                sender.send(Ok(item.clone())).await.ok();
+                            }
+                        }
+                        entry.or_insert(peer_info);
                     }
                     Message::SendAddrs(node_id, sender) => {
                         let id = last_id + 1;