rgw: introduce json_decode to easily decode json structures

Introducing a new way to define the json structure of an
object, similar to what we do with bufferlist encode/decode.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: decode RGWUserInfo using json_decode

RGWUserInfo can be decoded using json_decode, and fix
the ->dump() method to match that decoding.

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

Now testing json_decode functionality.

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