feat/web (Patchset 2) (#444)

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 
```

easytier/src/proto/mod.rs

@@ -5,6 +5,7 @@ pub mod cli;
 pub mod common;
 pub mod error;
 pub mod peer_rpc;
+pub mod web;
 
 #[cfg(test)]
 pub mod tests;