Support in DistributedCache to share cache files with other users after HADOOP-4493

Fixed DistributedCache to support sharing of the local cache files with other users on the same TaskTracker. The cache files are checked at the client side for public/private access on the file system, and that information is passed in the configuration. The TaskTrackers look at the configuration for each file during task localization, and, if the file was public on the filesystem, they are localized to a common space for sharing by all users' tasks on the TaskTracker. Else the file is localized to the user's private directory on the local filesystem.