fix(iroh): Reduce log-level of unknown pong message (#3242)

## Description

Turns out we get this normally when they are just delayed due to
saturated traffic.  That's not something that's worth spamming WARN
all over the placed for.

## Breaking Changes

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including how to migrate older code. -->

## Notes & open questions

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## Change checklist
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- [x] Self-review.

Author: flub

iroh/src/magicsock/node_map/node_state.rs

@@ -898,8 +898,10 @@ impl NodeState {
         let is_relay = src.is_relay();
         match self.sent_pings.remove(&m.tx_id) {
             None => {
-                // This is not a pong for a ping we sent.
-                warn!(tx = %HEXLOWER.encode(&m.tx_id), "received pong with unknown transaction id");
+                // This is not a pong for a ping we sent.  In reality however we probably
+                // did send this ping but it has timed-out by the time we receive this pong
+                // so we removed the state already.
+                debug!(tx = %HEXLOWER.encode(&m.tx_id), "received unknown pong (did it timeout?)");
                 None
             }
             Some(sp) => {