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/Dispatcher.h

@@ -99,6 +99,13 @@ class DispatcherBase {
   std::condition_variable m_reconnect_cv;
   unsigned int m_reconnect_proto_rev = 0x0300;
   bool m_do_reconnect = true;
+
+  // Condition variables for shutdown
+  std::mutex m_shutdown_mutex;
+  std::condition_variable m_dispatch_shutdown_cv;
+  std::condition_variable m_clientserver_shutdown_cv;
+  bool m_dispatch_shutdown = false;
+  bool m_clientserver_shutdown = false;
 };
 
 class Dispatcher : public DispatcherBase {