Java: call JNI AttachCurrentThread less frequently.

Each call to AttachCurrentThread results in a new Java thread object being
created.  This is inefficient and also causes debugging issues with Eclipse
due to constant creation and removal of threads.  Now AttachCurrentThread is
only called once for (all) listeners and once for logging (if used).

include/ntcore_cpp.h

@@ -180,6 +180,9 @@ void Flush();
  * Callback Creation Functions
  */
 
+void SetListenerOnStart(std::function<void()> on_start);
+void SetListenerOnExit(std::function<void()> on_exit);
+
 typedef std::function<void(unsigned int uid, StringRef name,
                            std::shared_ptr<Value> value,
                            unsigned int flags)> EntryListenerCallback;