cls/user: accounting functionality

Keep user total accounting in omap header, handle add/dec functionality.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
cls/user: a new op to retrieve user header

user header holds user total stats

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw, radosgw-admin, cls/user: retrieve user stats

add a new op to retrieve user header, new radosgw-admin command to stat
user.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>