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

Author: zsxwing <zsxwing@gmail.com>

Closes #3734 from zsxwing/SPARK-4883 and squashes the following commits:

e6f2b61 [zsxwing] Fix the name
cc74727 [zsxwing] Add a name to the directoryCleaner thread
[Minor] Improve some code in BroadcastTest for short

Using
    val arr1 = (0 until num).toArray
instead of
    val arr1 = new Array[Int](num)
    for (i <- 0 until arr1.length) {
      arr1(i) = i
    }
for short.

Author: carlmartin <carlmartinmax@gmail.com>

Closes #3750 from SaintBacchus/BroadcastTest and squashes the following commits:

43adb70 [carlmartin] Improve some code in BroadcastTest for short