mutex implemented for callback to racket.

Needs to be serialized and protected.

rktwebview.cpp

@@ -313,6 +313,12 @@ void rkt_webview_cleanup()
 
         handler->verbose = true;
         handler->event_queue->enqueue(EVT_NULL);
+
+        // I finally found out that this second is not important.
+        // The call to racket must be in a mutex in order to make sure
+        // this side of the library isn't cleaned before the asynchronous
+        // racket call returns. We need to serialize it. This has now
+        // been done using the evt_cb_mutex.
         //sleep_ms(1000);     // Why is this second so important?
         // I just don't get why the evt_guard thread is blocking and getting core dumped
         // on the thread join, if this second is not given