rgw: separate certain request info into different structure

This is needed for later request forwarding.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: some code cleanup

move s3 signing code to a different common file.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: rest_client, forward requests

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