authdb = function(callback) {
    db.request({
        type: "GET",
        url: "/_session"
    },
    function (err, resp) {
        if (err) {
            return callback(err, null);
        }
        callback(null, resp.info.authentication_db);
    });
}
Returns the authentication database for the current user's session.

@name authdb(callback)
@param {Function} callback
@api private