[CORE]codeStyle: uniform ConcurrentHashMap define in StorageLevel.scala with other places

Author: Zhang, Liye <liye.zhang@intel.com>

Closes #2793 from liyezhang556520/uniformHashMap and squashes the following commits:

5884735 [Zhang, Liye] [CORE]codeStyle: uniform ConcurrentHashMap define in StorageLevel.scala