[tcp] Set shutdownTimestamp before queue.shutdown()

In the writer thread, we use shutdownTimestamp to determine if we
should break out of the writer loop. If we set
shutdownTimestamp *after* queue.shutdown(), then there is a brief
period where nextStreamElement() returns 'null' but done() still
returns 'true'. Hence, we switch the logic "setting" those values.

smack-tcp/src/main/java/org/jivesoftware/smack/tcp/XMPPTCPConnection.java

@@ -1305,8 +1305,8 @@ public class XMPPTCPConnection extends AbstractXMPPConnection {
          */
         void shutdown(boolean instant) {
             instantShutdown = instant;
-            queue.shutdown();
             shutdownTimestamp = System.currentTimeMillis();
+            queue.shutdown();
         }
 
         /**