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/Notifier.cpp

@@ -10,12 +10,17 @@
 using namespace nt;
 
 ATOMIC_STATIC_INIT(Notifier)
+bool Notifier::s_destroyed = false;
 
 Notifier::Notifier() {
   m_active = false;
+  s_destroyed = false;
 }
 
-Notifier::~Notifier() { Stop(); }
+Notifier::~Notifier() {
+  s_destroyed = true;
+  Stop();
+}
 
 void Notifier::Start() {
   {