improve hole punching and stun test (#124)

* implement new stun test algorithm, do test faster and provide more info
* support punching for symmetric

easytier/src/common/defer.rs

@@ -0,0 +1,24 @@
+#[doc(hidden)]
+pub struct Defer<F: FnOnce()> {
+    // internal struct used by defer! macro
+    func: Option<F>,
+}
+
+impl<F: FnOnce()> Defer<F> {
+    pub fn new(func: F) -> Self {
+        Self { func: Some(func) }
+    }
+}
+
+impl<F: FnOnce()> Drop for Defer<F> {
+    fn drop(&mut self) {
+        self.func.take().map(|f| f());
+    }
+}
+
+#[macro_export]
+macro_rules! defer {
+	( $($tt:tt)* ) => {
+		let _deferred = $crate::common::defer::Defer::new(|| { $($tt)* });
+	};
+}