fix dns query (#864)

1. dns resolver should be global unique so dns cache can work. avoid dns query influence hole punching.
2. when system dns failed, fallback to hickory dns.

easytier/src/tunnel/common.rs

@@ -401,8 +401,8 @@ pub(crate) async fn wait_for_connect_futures<Fut, Ret, E>(
     mut futures: FuturesUnordered<Fut>,
 ) -> Result<Ret, TunnelError>
 where
-    Fut: Future<Output = Result<Ret, E>> + Send + Sync,
-    E: std::error::Error + Into<TunnelError> + Send + Sync + 'static,
+    Fut: Future<Output = Result<Ret, E>> + Send,
+    E: std::error::Error + Into<TunnelError> + Send + 'static,
 {
     // return last error
     let mut last_err = None;