udp: distribute announce swarm responses among socket workers

They don't have to be sent from the same worker that received the
request, so we can decrease performance loss from underutilized
threads this way.

crates/udp/src/common.rs

@@ -171,13 +171,17 @@ impl ConnectedRequestSender {
 
 pub struct ConnectedResponseSender {
     senders: Vec<thingbuf::mpsc::blocking::Sender<ConnectedResponseWithAddr, Recycler>>,
+    to_any_last_index_picked: usize,
 }
 
 impl ConnectedResponseSender {
     pub fn new(
         senders: Vec<thingbuf::mpsc::blocking::Sender<ConnectedResponseWithAddr, Recycler>>,
     ) -> Self {
-        Self { senders }
+        Self {
+            senders,
+            to_any_last_index_picked: 0,
+        }
     }
 
     pub fn try_send_ref_to(
@@ -193,6 +197,23 @@ impl ConnectedResponseSender {
     ) -> Result<SendRef<ConnectedResponseWithAddr>, thingbuf::mpsc::errors::Closed> {
         self.senders[index.0].send_ref()
     }
+
+    pub fn send_ref_to_any(
+        &mut self,
+    ) -> Result<SendRef<ConnectedResponseWithAddr>, thingbuf::mpsc::errors::Closed> {
+        let start = self.to_any_last_index_picked + 1;
+
+        for i in (start..start + self.senders.len()).map(|i| i % self.senders.len()) {
+            if let Ok(sender) = self.senders[i].try_send_ref() {
+                self.to_any_last_index_picked = i;
+
+                return Ok(sender);
+            }
+        }
+
+        self.to_any_last_index_picked = start % self.senders.len();
+        self.send_ref_to(SocketWorkerIndex(self.to_any_last_index_picked))
+    }
 }
 
 pub type ConnectedResponseReceiver =