rgw: hook up delete-marker detection

rgw: hook up delete-marker detection

Introduces is_delete_marker() getter in rgw::sal::Object interface,
and implements it for rgw/sal/rados (and filter), replacing
unimplemented Object::get_delete_marker().

(Looked into
rgw/sal/posix, but delete marker handling seems not working as expected
there, so deferring for now.)

Next, the new interface is used in GET/HEAD request paths to return
a correct AWS-specified x-amz-delete-marker header when required, c.f.
https://docs.aws.amazon.com/AmazonS3/latest/userguide/DeleteMarker.html.

Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>