perf(l1): reusing FindNode message per lookup loop (#5047)

**Motivation**
Noticed that random key creation for FindNode messages was consuming a
lot of CPU:
<img width="1860" height="563" alt="image"
src="https://github.com/user-attachments/assets/9d823161-6013-41fe-94b1-008803edb353"
/>

**Description**
So, instead of creating a new key on each message, we create a FindNode
message on every lookup cycle, and encode it a single time to send to
all peers. It removes some randomness but gains in performance. In any
case, next lookup cycle will create a new key.

New flamegraph:
<img width="1854" height="539" alt="image"
src="https://github.com/user-attachments/assets/d2b42609-e178-4ec7-a1a7-31230926c06e"
/>

Tested on mainnet to see if it reduced peer finding performance, and it
took 3' 24'' to find 50 peers
```
2025-10-24T18:48:42.213697Z  INFO ethrex_p2p::network: P2P Snap Sync:
elapsed: 00h 03m 23s 00ms
50 peers.
Current step: Downloading Headers
```
and 13' 43'' to reach the 100 peers limit:
```
2025-10-24T18:59:02.280007Z  INFO ethrex_p2p::network: P2P Snap Sync:
elapsed: 00h 13m 43s 00ms
100 peers.
Current step: Downloading Headers
``` 

---------

Co-authored-by: Edgar <[email protected]>

Author: ElFantasma

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## Perf
 
+### 2025-10-27
+
+- Reusing FindNode message per lookup loop instead of randomizing the key for each message. [#5047](https://github.com/lambdaclass/ethrex/pull/5047)
+
 ### 2025-10-21
 
 - Instead of lazy computation of blocklist, do greedy computation of allowlist and store the result, fetch it with the DB. [#4961](https://github.com/lambdaclass/ethrex/pull/4961)

crates/networking/p2p/discv4/server.rs

@@ -216,8 +216,18 @@ impl DiscoveryServer {
     }
 
     async fn lookup(&mut self) -> Result<(), DiscoveryServerError> {
+        // Sending the same FindNode message to all contacts to optimize message creation and encoding
+        let expiration: u64 = get_msg_expiration_from_seconds(EXPIRATION_SECONDS);
+        let random_priv_key = SecretKey::new(&mut OsRng);
+        let random_pub_key = public_key_from_signing_key(&random_priv_key);
+        let msg = Message::FindNode(FindNodeMessage::new(random_pub_key, expiration));
+        let mut buf = BytesMut::new();
+        msg.encode_with_header(&mut buf, &self.signer);
+
         for contact in self.peer_table.get_contacts_for_lookup().await? {
-            if self.send_find_node(&contact.node).await.is_err() {
+            if self.udp_socket.send_to(&buf, &contact.node.udp_addr()).await.inspect_err(
+                |e| error!(sending = ?msg, addr = ?&contact.node.udp_addr(), err=?e, "Error sending message"),
+            ).is_err() {
                 self.peer_table
                     .set_disposable(&contact.node.node_id())
                     .await?;
@@ -309,20 +319,6 @@ impl DiscoveryServer {
         Ok(())
     }
 
-    async fn send_find_node(&self, node: &Node) -> Result<(), DiscoveryServerError> {
-        let expiration: u64 = get_msg_expiration_from_seconds(EXPIRATION_SECONDS);
-
-        let random_priv_key = SecretKey::new(&mut OsRng);
-        let random_pub_key = public_key_from_signing_key(&random_priv_key);
-
-        let msg = Message::FindNode(FindNodeMessage::new(random_pub_key, expiration));
-        self.send(msg, node.udp_addr()).await?;
-
-        debug!(sent = "FindNode", to = %format!("{:#x}", node.public_key));
-
-        Ok(())
-    }
-
     async fn send_neighbors(
         &self,
         neighbors: Vec<Node>,