introduce peer center (#13)

peer_center is used to collect peer info into one peer node.

the center node is selected with the following rules:

1. has smallest peer id
2. TODO: has allow_to_be_center peer feature

peer center is not guaranteed to be stable and can be changed when peer enter
or leave.  it's used to reduce the cost to exchange infos between peers.

easytier-core/src/peers/rpc_service.rs

@@ -1,5 +1,6 @@
 use std::sync::Arc;
 
+use easytier_rpc::cli::PeerInfo;
 use easytier_rpc::peer_manage_rpc_server::PeerManageRpc;
 use easytier_rpc::{ListPeerRequest, ListPeerResponse, ListRouteRequest, ListRouteResponse};
 use tonic::{Request, Response, Status};
@@ -14,17 +15,10 @@ impl PeerManagerRpcService {
     pub fn new(peer_manager: Arc<PeerManager>) -> Self {
         PeerManagerRpcService { peer_manager }
     }
-}
-
-#[tonic::async_trait]
-impl PeerManageRpc for PeerManagerRpcService {
-    async fn list_peer(
-        &self,
-        _request: Request<ListPeerRequest>, // Accept request of type HelloRequest
-    ) -> Result<Response<ListPeerResponse>, Status> {
-        let mut reply = ListPeerResponse::default();
 
+    pub async fn list_peers(&self) -> Vec<PeerInfo> {
         let peers = self.peer_manager.get_peer_map().list_peers().await;
+        let mut peer_infos = Vec::new();
         for peer in peers {
             let mut peer_info = easytier_rpc::PeerInfo::default();
             peer_info.peer_id = peer.to_string();
@@ -38,7 +32,24 @@ impl PeerManageRpc for PeerManagerRpcService {
                 peer_info.conns = conns;
             }
 
-            reply.peer_infos.push(peer_info);
+            peer_infos.push(peer_info);
+        }
+
+        peer_infos
+    }
+}
+
+#[tonic::async_trait]
+impl PeerManageRpc for PeerManagerRpcService {
+    async fn list_peer(
+        &self,
+        _request: Request<ListPeerRequest>, // Accept request of type HelloRequest
+    ) -> Result<Response<ListPeerResponse>, Status> {
+        let mut reply = ListPeerResponse::default();
+
+        let peers = self.list_peers().await;
+        for peer in peers {
+            reply.peer_infos.push(peer);
         }
 
         Ok(Response::new(reply))