Remove unnecessary member variable mConfigFN
Create Rados instance with a user name retrieved from the conf file

For this, the order in which the instance was created and the configuration
file was read had to be changed.
Cosmetic change to replace tabs with spaces
Explain in the README file how to configure the user name for Rados
Use only one xattr for storing the mode, uid and gid

This way it is faster than having 3 dedicated xattrs.