refactor(iroh): Rename the relay-is-ready-to-send waker (#3014)

## Description

I thought I was going to put in a queue or channel of some sort... but
not yet.  Still, let's rename this for now.

## Breaking Changes

<!-- Optional, if there are any breaking changes document them,
including how to migrate older code. -->

## Notes & open questions

<!-- Any notes, remarks or open questions you have to make about the PR.
-->

## Change checklist

- [x] Self-review.
- [x] 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.
- [x] Tests if relevant.
- [x] All breaking changes documented.

iroh/src/magicsock.rs

@@ -187,8 +187,11 @@ pub(crate) struct MagicSock {
     /// [`AsyncUdpSocket`].  This queue takes care of the wakers needed by
     /// [`AsyncUdpSocket::poll_recv`].
     relay_datagrams_queue: Arc<RelayDatagramsQueue>,
-
-    network_send_wakers: Arc<parking_lot::Mutex<Option<Waker>>>,
+    /// Waker to wake the [`AsyncUdpSocket`] when more data can be sent to the relay server.
+    ///
+    /// This waker is used by [`IoPoller`] and the [`RelayActor`] to signal when more
+    /// datagrams can be sent to the relays.
+    relay_send_waker: Arc<parking_lot::Mutex<Option<Waker>>>,
     /// Counter for ordering of [`MagicSock::poll_recv`] polling order.
     poll_recv_counter: AtomicUsize,
 
@@ -453,7 +456,7 @@ impl MagicSock {
             ipv4_poller,
             ipv6_poller,
             relay_sender,
-            relay_send_waker: self.network_send_wakers.clone(),
+            relay_send_waker: self.relay_send_waker.clone(),
         })
     }
 
@@ -1549,7 +1552,7 @@ impl Handle {
             closing: AtomicBool::new(false),
             closed: AtomicBool::new(false),
             relay_datagrams_queue: relay_datagrams_queue.clone(),
-            network_send_wakers: Arc::new(parking_lot::Mutex::new(None)),
+            relay_send_waker: Arc::new(parking_lot::Mutex::new(None)),
             poll_recv_counter: AtomicUsize::new(0),
             actor_sender: actor_sender.clone(),
             ipv6_reported: Arc::new(AtomicBool::new(false)),

iroh/src/magicsock/relay_actor.rs

@@ -427,7 +427,7 @@ impl RelayActor {
         }
 
         // Wake up the send waker if one is waiting for space in the channel
-        let mut wakers = self.msock.network_send_wakers.lock();
+        let mut wakers = self.msock.relay_send_waker.lock();
         if let Some(waker) = wakers.take() {
             waker.wake();
         }