Add setXAttr method to RadosFsInfo
Add getXAttr method to RadosFsInfo
Add removeXAttr method to RadosFsInfo
Add getXAttrsMap method to RadosFsInfo
test: Add unit tests for the RadosFsInfo's xattrs methods
Override stat in RadosFsDir

Because this way we might avoid having to really stat the dir
and use the cached stat instead.
Override stat in RadosFsFile

Because this way we might avoid having to really stat the file
and use the instance's stat instead.