swarm-nats-auth: answer callout requests concurrently, skip expired ones

Each request now runs in its own task, at most CONCURRENT_REQUESTS (32)
at once, instead of being awaited inline by the subscription loop. A
request whose exp (nats-server sets it to send time plus its
authorization.timeout) has passed is skipped without a lookup or reply.

Fixes #5434.

swarm-nats-auth/src/xkey.rs

@@ -240,7 +240,7 @@ pub(crate) mod tests {
         let (server_key, opened) = open(&own, Some(&server.public_key()), &sealed).expect("opens");
         assert_eq!(server_key.public_key(), server.public_key());
         let req = crate::request::decode(&opened).expect("a request");
-        assert_eq!(req.user_nkey, "UCLIENT");
+        assert_eq!(req.nats.user_nkey, "UCLIENT");
 
         let issuer = nkeys::KeyPair::new_account();
         let permissions = crate::policy::Permissions {
@@ -251,8 +251,8 @@ pub(crate) mod tests {
         let token = crate::respond::grant(
             &issuer,
             "APP",
-            &req.server_id.id,
-            &req.user_nkey,
+            &req.nats.server_id.id,
+            &req.nats.user_nkey,
             &permissions,
             None,
         );