rgw: send data back when copying object

Currently doing it only when copying between regions. This is
needed so that the operation doesn't time out (as it can take
a long time and the web server may just hang on us since we're
not sending any data).
This is configurable and can be disabled. Currently only implemented
for S3.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
rgw: dump progress through swift object copy

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

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
PendingReleaseNotes: update about new rgw copy obj response

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