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-web/src/client_manager/storage.rs

@@ -0,0 +1,72 @@
+use std::sync::{Arc, Weak};
+
+use dashmap::{DashMap, DashSet};
+
+// use this to maintain Storage
+#[derive(Debug, Clone, serde::Serialize, serde::Deserialize)]
+pub struct StorageToken {
+    pub token: String,
+    pub client_url: url::Url,
+    pub machine_id: uuid::Uuid,
+}
+
+#[derive(Debug)]
+pub struct StorageInner {
+    // some map for indexing
+    pub token_clients_map: DashMap<String, DashSet<url::Url>>,
+    pub machine_client_url_map: DashMap<uuid::Uuid, url::Url>,
+}
+
+#[derive(Debug, Clone)]
+pub struct Storage(Arc<StorageInner>);
+pub type WeakRefStorage = Weak<StorageInner>;
+
+impl TryFrom<WeakRefStorage> for Storage {
+    type Error = ();
+
+    fn try_from(weak: Weak<StorageInner>) -> Result<Self, Self::Error> {
+        weak.upgrade().map(|inner| Storage(inner)).ok_or(())
+    }
+}
+
+impl Storage {
+    pub fn new() -> Self {
+        Storage(Arc::new(StorageInner {
+            token_clients_map: DashMap::new(),
+            machine_client_url_map: DashMap::new(),
+        }))
+    }
+
+    pub fn add_client(&self, stoken: StorageToken) {
+        let inner = self
+            .0
+            .token_clients_map
+            .entry(stoken.token)
+            .or_insert_with(DashSet::new);
+        inner.insert(stoken.client_url.clone());
+
+        self.0
+            .machine_client_url_map
+            .insert(stoken.machine_id, stoken.client_url.clone());
+    }
+
+    pub fn remove_client(&self, stoken: &StorageToken) {
+        self.0.token_clients_map.remove_if(&stoken.token, |_, set| {
+            set.remove(&stoken.client_url);
+            set.is_empty()
+        });
+
+        self.0.machine_client_url_map.remove(&stoken.machine_id);
+    }
+
+    pub fn weak_ref(&self) -> WeakRefStorage {
+        Arc::downgrade(&self.0)
+    }
+
+    pub fn get_client_url_by_machine_id(&self, machine_id: &uuid::Uuid) -> Option<url::Url> {
+        self.0
+            .machine_client_url_map
+            .get(&machine_id)
+            .map(|url| url.clone())
+    }
+}