rgw: show mtime for metadata entries

currently only shows for buckets

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: preserve bucket creation time across different zones / regions

Keep bucket creation time in RGWBucketInfo, and make use
of it where necessary.

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