This patch implements:
1. A dns server that handles .et.net. zone in local and forward all other queries to system dns server.
2. A dns server instance which is a singleton in one machine, using one specific tcp port to be exclusive with each other. this instance is responsible for config system dns and run the dns server to handle dns queries.
3. A dns client instance that all easytier instance will run one, this instance will try to connect to dns server instance, and update the dns record in the dns server instance.
this pr only implements the system config for windows. linux & mac will do later.
This patch implement a restful server without any auth.
usage:
```bash
# run easytier-web, which acts as an gateway and registry for all easytier-core
$> easytier-web
# run easytier-core and connect to easytier-web with a token
$> easytier-core --config-server udp://127.0.0.1:22020/fdsafdsa
# use restful api to list session
$> curl -H "Content-Type: application/json" -X GET 127.0.0.1:11211/api/v1/sessions
[{"token":"fdsafdsa","client_url":"udp://127.0.0.1:48915","machine_id":"de3f5b8f-0f2f-d9d0-fb30-a2ac8951d92f"}]%
# use restful api to run a network instance
$> curl -H "Content-Type: application/json" -X POST 127.0.0.1:11211/api/v1/network/de3f5b8f-0f2f-d9d0-fb30-a2ac8951d92f -d '{"config": "listeners = [\"udp://0.0.0.0:12344\"]"}'
# use restful api to get network instance info
$> curl -H "Content-Type: application/json" -X GET 127.0.0.1:11211/api/v1/network/de3f5b8f-0f2f-d9d0-fb30-a2ac8951d92f/65437e50-b286-4098-a624-74429f2cb839
```
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.
