Add immediate_notify and is_new to entry listener.

On the callback function, is_new indicates the value is newly added.

On adding a callback function, immediate_notify indicates the callback
should be called once (with is_new=true) for each matching entry that
already exists.

src/ntcore_cpp.cpp

@@ -61,8 +61,8 @@ void Flush() {
  * Callback Creation Functions
  */
 
-unsigned int AddEntryListener(StringRef prefix,
-                              EntryListenerCallback callback) {
+unsigned int AddEntryListener(StringRef prefix, EntryListenerCallback callback,
+                              bool immediate_notify) {
   return 0;
 }