optimize Mailbox._status usage

- make Scheduled==4 to allow bitwise operations
- unfold become(status) into three methods for better inlining
- optimize patterns in setAsIdle away: table switch is probably slower
  than a single conditional
- fix LoggingReceiveSpec glitch wrt. async stop()

akka-actor/src/main/java/akka/dispatch/AbstractMailbox.java

@@ -7,6 +7,6 @@ package akka.dispatch;
 import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
 
 abstract class AbstractMailbox {
-    private volatile int _status = Mailbox.Idle();
+    private volatile int _status; // not initialized because this is faster: 0 == Open
     protected final static AtomicIntegerFieldUpdater<AbstractMailbox> updater = AtomicIntegerFieldUpdater.newUpdater(AbstractMailbox.class, "_status");
-}
+}