Comment 8 for bug 1182628

This is still a problem, and I've at least gotten so far as writing a failing probe test for it: https://review.opendev.org/c/openstack/swift/+/803406

I think we can (ab)use `Expect: 100-continue` and `Transfer-Encoding: chunked` to get a two-phase commit like we've got for PUTs with `If-None-Match: *` -- but it's going to require a bunch of plumbing.