[SPARK-4883][Shuffle] Add a name to the directoryCleaner thread

Actually, this should be daemon, I think, otherwise it may prevent the JVM from exiting even when it's not doing anything. If we wanted to prevent the service from exiting before cleanup has completed, I would rather block in a stop() method until the queue of directories to delete has been drained, but I do not think this too important. Unlike Spark Executors, the graceful termination of this service should be uncorrelated with buildup of shuffle data.