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/common/mod.rs

@@ -11,6 +11,7 @@ pub mod compressor;
 pub mod config;
 pub mod constants;
 pub mod defer;
+pub mod dns;
 pub mod error;
 pub mod global_ctx;
 pub mod ifcfg;