Java: Improve robustness against JVM crashes on exit.

The JVM doesn't always do a good job of telling JNI modules that the JVM
is going away, which results in a crash in the JavaGlobal and/or
JavaWeakGlobal destructors as they try to delete the associated references
after the JVM has already gone away.

To protect against this, the Notifier now has a static variable that's set
when the Notifier instance (a singleton) is destroyed.  This is used by
JavaGlobal and JavaWeakGlobal to detect when a process exit is in process.

src/ntcore_c.cpp

@@ -200,6 +200,8 @@ void NT_RemoveConnectionListener(unsigned int conn_listener_uid) {
   nt::RemoveConnectionListener(conn_listener_uid);
 }
 
+int NT_NotifierDestroyed() { return nt::NotifierDestroyed(); }
+
 /*
  * Remote Procedure Call Functions
  */