rgw: more json encoding/decoding changes

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw_json_enc: user info encoding changes

Retain old encoding, to keep old versions compatibility.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: complete json decoding for user info

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

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