rgw: don't copy object when it's copied into itself

Fixes: #4150
Backport: bobtail

When object copied into itself, object will not be fully copied: tail
reference count stays the same, head part is rewritten.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>
rgw: fix multipart uploads listing

Fixes: #4177
Backport: bobtail
Listing multipart uploads had a typo, and was requiring the
wrong resource (uploadId instead of uploads).

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>