return the cached response's body if it had a newer `Last-Modified` date.
 *  Fix: Update the stored timestamp on conditional cache hits.
 *  New: Optimized HTTP/2 request header encoding. More headers are HPACK-encoded and string
    literals are now Huffman-encoded.
 *  New: Expose `Part` headers and body in `Multipart`.

## Ztunnel Shutdown/Upgrade/Restart

This could be upgrading to a new version, or changing some part of the DaemonSet spec, etc.

### Ztunnel Shutdown Requirements

Our high level requirement is to minimize traffic disruption.

However, because Ztunnel operates only at Layer 4, we are in a tricky position:
* TCP is stateful, so we cannot really just pass state over to the other process.

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