fix: force-closing local discovery TCP server could always time out (#498)

We have a way to force-close the local discovery TCP server. This was
susceptible to the following race condition bug:

1. A connection opens and is established.

2. A duplicate connection opens. [We decide to keep the new one. We
   start to close the old connection and enqueue a "promotion" of the
   new one.][2]

3. We close the TCP server. This does not immediately halt the server;
   it ["stops the server from accepting new connections and keeps
   existing connections"][docs].

   [We iterate through all existing connections and destroy them][3],
   but the duplicate connection is not yet in this list so it is never
   destroyed and the server will never close.

4. The old connection is closed and ["promotion" of the new connection
   completes][4]. It is never destroyed.

This caused [intermittent CI failures][ci] and should be fixed by this
change.

[docs]: https://nodejs.org/docs/latest-v18.x/api/net.html#serverclosecallback
[ci]: https://github.com/digidem/mapeo-core-next/actions/runs/7996074898/job/21837815901
[2]: https://github.com/digidem/mapeo-core-next/blob/5e05e35200297435f4ebd33df2c5b1444ed4ef1b/src/discovery/local-discovery.js#L229-L232
[3]: https://github.com/digidem/mapeo-core-next/blob/5e05e35200297435f4ebd33df2c5b1444ed4ef1b/src/discovery/local-discovery.js#L278-L280
[4]: https://github.com/digidem/mapeo-core-next/blob/5e05e35200297435f4ebd33df2c5b1444ed4ef1b/src/discovery/local-discovery.js#L173

Author: EvanHahn

src/discovery/local-discovery.js

@@ -226,6 +226,16 @@ export class LocalDiscovery extends TypedEmitter {
         return
       }
     }
+
+    // Bail if the server has already stopped by this point. This should be
+    // rare but can happen during connection swaps if the new connection is
+    // "promoted" after the server's doors have already been closed.
+    if (!this.#server.listening) {
+      this.#log('server stopped, destroying connection %h', remotePublicKey)
+      conn.destroy()
+      return
+    }
+
     this.#noiseConnections.set(remoteId, conn)
 
     conn.on('close', () => {