Isolate datagram proxy state and replies between instances and peers

src/utun_instance.h

@@ -248,6 +248,8 @@ struct UTUN_INSTANCE {
 
     // TCP proxy server (exit node)
     struct tcp_proxy_server tcp_proxy_server;
+    struct udp_proxy_ctx* udp_proxy;
+    struct icmp_proxy_ctx* icmp_proxy;
 
     // Networks (queue of NETWORK_ENTRY, indexed by 56-bit id)
     struct ll_queue* networks;