Some devices have ipv6 but don't allow input connection, this patch add hole punching for these devices.
- **add v6 hole punch msg to udp tunnel**
- **send hole punch packet when do ipv6 direct connect**
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.
* support ipv6 stun
* show interface and public ip in cli node info
* direct conn should keep trying unless already direct connected
* peer should use conn with smallest latency
* deprecate ipv6_listener, use -l instead
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
This patch removes Tarpc & Tonic GRPC and implements a customized rpc framework, which can be used by peer rpc and cli interface.
web config server can also use this rpc framework.
moreover, rewrite the public server logic, use ospf route to implement public server based networking. this make public server mesh possible.
