Improve PromiseActorRef as per team comments

akka-actor/src/main/java/akka/pattern/AbstractPromiseActorRef.java

@@ -0,0 +1,19 @@
+/**
+ * Copyright (C) 2009-2012 Typesafe Inc. <http://www.typesafe.com>
+ */
+
+package akka.pattern;
+
+import akka.util.Unsafe;
+
+final class AbstractPromiseActorRef {
+    final static long stateOffset;
+
+    static {
+        try {
+            stateOffset = Unsafe.instance.objectFieldOffset(PromiseActorRef.class.getDeclaredField("_stateDoNotCallMeDirectly"));
+        } catch(Throwable t){
+            throw new ExceptionInInitializerError(t);
+        }
+    }
+}