rgw: turn swift COPY into PUT

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: get rid of req_state.bucket_name

No need for this field, as we already have req_state.bucket_name_str.
This saves us some memory allocation / freeing through every request
processing.

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