nat4-nat4 punch (#388)

this patch optimize the udp hole punch logic:

1. allow start punch hole before stun test complete.
2. add lock to symmetric punch, avoid conflict between concurrent hole punching task.
3. support punching hole for predictable nat4-nat4.
4. make backoff of retry reasonable

easytier/src/tunnel/mod.rs

@@ -94,7 +94,7 @@ pub trait Tunnel: Send {
 
 #[auto_impl::auto_impl(Arc)]
 pub trait TunnelConnCounter: 'static + Send + Sync + Debug {
-    fn get(&self) -> u32;
+    fn get(&self) -> Option<u32>;
 }
 
 #[derive(Debug, Clone, Copy, PartialEq)]
@@ -114,8 +114,8 @@ pub trait TunnelListener: Send {
         #[derive(Debug)]
         struct FakeTunnelConnCounter {}
         impl TunnelConnCounter for FakeTunnelConnCounter {
-            fn get(&self) -> u32 {
-                0
+            fn get(&self) -> Option<u32> {
+                None
             }
         }
         Arc::new(Box::new(FakeTunnelConnCounter {}))