Some changes in FileSystem class.

Removed the static default file system field. It was thread-hostile. (The
file system now has to be provided explicitly by clients.) Changed the way the
logger is accessed, do not call a non-final, non-private method in
constructor. Member fields have been made volatile, they might be accessed by
different threads.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1527248 13f79535-47bb-0310-9956-ffa450edef68
DefaultFileSystem now uses the logger defined by the base class.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1527249 13f79535-47bb-0310-9956-ffa450edef68