The new ->zmore and ->zflush APIs make it possible to replace
existing verbose usages of Compress::Raw::Deflate and simplify
buffering logic for streaming large gzipped data.
One potentially user visible change is we now break the mbox.gz
response on zlib failures, instead of silently continuing onto
the next message. zlib only seems to fail on OOM, which should
be rare; so it's ideal we drop the connection anyways.