rgw: metadata set/get infrastructure

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: metadata manager, api to list keys

Also, implement key listing for user metadata.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: put metadata, other cleanups

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: add top level metadata handler

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: user metadata updates also key version

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: metadata list user, only show uids

don't show unrelated object names

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