rename refresh() to _new()

a session CANNOT be refreshed.
The reason of the whole excercise it to guarantee that all sessions created will have the same callback functions registered on the engine (the ones from the CallbackDispatcher)

src/Adapter.cc

@@ -65,7 +65,7 @@ namespace pEp {
             ::adapter_group_init();
         }
 
-        void Session::refresh()
+        void Session::_new()
         {
             std::lock_guard<mutex> lock(mut);