rgw: intra-region copy, preserve mtime

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: fix inter-region copy (ofs wasn't correct)

Fixing an issue with data read. Since inter-region copy uses
embedded metadata within the data input, we need to fix the ofs
accordingly.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: object mtime the same for both object and bucket index

Make sure object mtime that's passed to bucket index is the
same as the one set on the object.

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