Move generic gzip output operations to private helper for easier sharing of capabilities between GzipOutputStream and GzipAsyncOutputStream.

 - Makes it possible to decompress data granularly via push-based API.
 - Adds ability to flush output at an arbitrary point.
 - Removes manual `ended` check as zlib checks it for us.

Unfortunately TCP does not always distinguish between dirty connection failure and clean connection shutdown. In practice, though, "Premature EOF" errors on streams basically always appear to be due to disconnects (including when the peer crashes).