control frame callbacks are non-const references:

fix #653

Actions Required:

* Modify calls to set the control frame callback, to
  pass non-const reference instances, and manage the
  lifetime of the instance.

test/websocket/doc_snippets.cpp

@@ -189,12 +189,13 @@ boost::asio::ip::tcp::socket sock{ios};
 {
     stream<boost::asio::ip::tcp::socket> ws{ios};
 //[ws_snippet_17
-    ws.control_callback(
+    auto cb =
         [](frame_type kind, string_view payload)
         {
             // Do something with the payload
             boost::ignore_unused(kind, payload);
-        });
+        };
+    ws.control_callback(cb);
 //]
 
 //[ws_snippet_18