JobTracker memory leak caused by CleanupQueue reopening FileSystem

When a job is completed, closeAllForUGI is called to close all the cached FileSystems in the FileSystem cache. However, the CleanupQueue may run after this occurs and call FileSystem.get() to delete the staging directory, adding a FileSystem to the cache that will never be closed.

People on the user-list have reported this causing their JobTrackers to OOME every two weeks.