rgw: add the remove-x-delete feature to cancel swift object expiration

rgw: add the remove-x-delete feature to cancel swift object expiration

In openstack swift, it also support the feature to cancel the object expiration,
which could be found at last point in
https://docs.openstack.org/user-guide/cli-swift-set-object-expiration.html. we
can remove the object expiration by set 'X-Remove-Delete-At:'.

This patch also could fix the bug that when we set the object expiration and
then upload the same object to the container again. The previous object expiration
also works, which is not compatible with the openstack swift.

Fixes: http://tracker.ceph.com/issues/19074
Signed-off-by: Jing Wenjun <jingwenjun@cmss.chinamobile.com>