Implement join with timeout (and detach).

Under certain situations (notably JNI shutdown), it's possible to get
deadlock when using thread join().  To avoid this, implement a timeout.
Normally we try to simply join the thread, but if it times out, we
detach the thread instead.

src/Notifier.h

@@ -101,6 +101,9 @@ class Notifier {
   std::queue<ConnectionNotification> m_conn_notifications;
 
   std::thread m_thread;
+  std::mutex m_shutdown_mutex;
+  std::condition_variable m_shutdown_cv;
+  bool m_shutdown = false;
 
   ATOMIC_STATIC_DECL(Notifier)
   static bool s_destroyed;