rpc: send ending packet in Server instead of UDPServer

udp_server might not be created due to error (occupied port etc.), in which case its destructor and thread-ending call chain will not be excuted in Server::Stop. However, the ending packet still need to be send no matter udp is on or not, so move it to Server::Stop

src/core/rpc/server.cpp

@@ -25,6 +25,7 @@ void Server::Start() {
 
 void Server::Stop() {
     udp_server.reset();
+    NewRequestCallback(nullptr); // Notify the RPC server to end
 }
 
 void Server::NewRequestCallback(std::unique_ptr<RPC::Packet> new_request) {