[SPARK-1097] Workaround Hadoop conf ConcurrentModification issue

Workaround Hadoop conf ConcurrentModification issue


@aarondav , Yes, I also thought about this before. The reason I keep use broadcastedConf.value.value here is because: though broadcast variable is suggested to be read only and not changed, But I wonder maybe in case someone miss use it and change the value, read the latest value might be helpful. And it read the latest code in the next line in the original code, so I keep this style. But think again, if the value did changed in some place without hold any synchronize lock, this might still not be able to solve the  problem. I will update the pull request.