Try harder to not let a node flap after the gossip goodbye.

Patch by brandonwilliams, reviewed by jasobrown for CASSANDRA-5254

src/java/org/apache/cassandra/gms/GossipDigestAck2VerbHandler.java

@@ -37,6 +37,13 @@ public class GossipDigestAck2VerbHandler implements IVerbHandler<GossipDigestAck
             InetAddress from = message.from;
             logger.trace("Received a GossipDigestAck2Message from {}", from);
         }
+        if (!Gossiper.instance.isEnabled())
+        {
+            if (logger.isTraceEnabled())
+                logger.trace("Ignoring GossipDigestAck2Message because gossip is disabled");
+            return;
+        }
+
         Map<InetAddress, EndpointState> remoteEpStateMap = message.payload.getEndpointStateMap();
         /* Notify the Failure Detector */
         Gossiper.instance.notifyFailureDetector(remoteEpStateMap);

src/java/org/apache/cassandra/gms/Gossiper.java

@@ -1077,7 +1077,7 @@ public class Gossiper implements IFailureDetectionEventListener, GossiperMBean
         logger.info("Announcing shutdown");
         try
         {
-            Thread.sleep(intervalInMillis);
+            Thread.sleep(intervalInMillis * 2);
         }
         catch (InterruptedException e)
         {