[SPARK-7236] [CORE] Fix to prevent AkkaUtils askWithReply from sleeping on final attempt

Added a check so that if `AkkaUtils.askWithReply` is on the final attempt, it will not sleep for the `retryInterval`.  This should also prevent the thread from sleeping for `Int.Max` when using `askWithReply` with default values for `maxAttempts` and `retryInterval`.

Author: Bryan Cutler <bjcutler@us.ibm.com>

Closes #5896 from BryanCutler/askWithReply-sleep-7236 and squashes the following commits:

653a07b [Bryan Cutler] [SPARK-7236] Fix to prevent AkkaUtils askWithReply from sleeping on final attempt

core/src/main/scala/org/apache/spark/util/AkkaUtils.scala

@@ -183,7 +183,9 @@ private[spark] object AkkaUtils extends Logging {
           lastException = e
           logWarning(s"Error sending message [message = $message] in $attempts attempts", e)
       }
-      Thread.sleep(retryInterval)
+      if (attempts < maxAttempts) {
+        Thread.sleep(retryInterval)
+      }
     }
 
     throw new SparkException(