Make it possible to find out if we're shutting down or not.

git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@5321 542714f4-19e9-0310-aa3c-eee0fc999fb1

src/java/com/threerings/presents/server/ShutdownManager.java

@@ -97,6 +97,14 @@ public class ShutdownManager
         });
     }
 
+    /**
+     * Returns true if we're in the process of shutting down.
+     */
+    public boolean isShuttingDown ()
+    {
+        return _downers == null;
+    }
+
     /**
      * Shuts down all shutdowners immediately on the caller's thread.
      */