rgw: key indexes are only link to user info

Instead of keeping multiple copies of the user info,
we just treat the key index as a pointer to the actual
user info (indexed by uid). This helps with two issues:
first, it scales better as we don't need to update the
entire set of keys whenever we make any change. Second,
it helps with the uid index atomicity.
One point to keep in mind is that both the links and the
info can be cached, so effect on performance is minimal.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
Reviewed-by: caleb miles <caleb.miles@inktank.com>