rgw: read bucket metadata before writing it

In order to keep track of version.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: slightly simplify metadata abstraction

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: tie bucket/user removal to mdlog differently

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