rgw: move some code around, no actual changes

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: extend infrastructure to handle user stats read

Some helper functions that return data in required format for quota, add
async functions.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: implement quota handlers for user stats

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

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