refactor: replace static with const for global constants (#494)

src/rendezvous_server.rs

@@ -58,7 +58,7 @@ type Sender = mpsc::UnboundedSender<Data>;
 type Receiver = mpsc::UnboundedReceiver<Data>;
 static ROTATION_RELAY_SERVER: AtomicUsize = AtomicUsize::new(0);
 type RelayServers = Vec<String>;
-static CHECK_RELAY_TIMEOUT: u64 = 3_000;
+const CHECK_RELAY_TIMEOUT: u64 = 3_000;
 static ALWAYS_USE_RELAY: AtomicBool = AtomicBool::new(false);
 
 #[derive(Clone)]