finish tcp rendezvous keep alive logic following mqtt, but defined by

server so that it can be easily to be controlled at server side.

libs/hbb_common/protos/rendezvous.proto

@@ -73,6 +73,7 @@ message RegisterPkResponse {
     SERVER_ERROR = 7;
   }
   Result result = 1;
+  int32 keep_alive = 2;
 }
 
 message PunchHoleResponse {

libs/hbb_common/src/tcp.rs

@@ -330,6 +330,9 @@ impl Encrypt {
     }
 
     pub fn dec(&mut self, bytes: &mut BytesMut) -> Result<(), Error> {
+        if bytes.is_empty() {
+            return Ok(());
+        }
         self.2 += 1;
         let nonce = FramedStream::get_nonce(self.2);
         match secretbox::open(bytes, &nonce, &self.0) {